The Eruption of Vesuvius in 1872

d. By measurements of the horizontal and vertical distances of overthrow or of projection, to infer either the velocity of projection, or angle of emergence.

Fractures by shock present their planes always nearly in directions transverse to the wave path. Projections or overthrow take place (unless secondarily disturbed) in the line of the wave path, or in the vertical plane passing through it: but the direction of fall or overthrow may be either in the same direction as the wave transit (i.e., as the motion of the wave particle in the first semiphase), or contrary to it.

It is thus obvious that the principal phenomena presented by the effects of earthquake shock upon the objects usually occurring upon the surface of the inhabited parts of the earth, resolve themselves into problems of three orders, and are all amenable to mechanical treatment, viz.:

Problems relating to the direction and amount of velocity producing fracture or fissures.

Problems relating to the single or multiplied oscillations of bodies, considered as compound pendulums.

Problems referable to the theory of projectiles.

These three may combine in several cases, and on the part of the observer must combine with measurements, angular and linear, and with geodetic operations to be conducted in the shaken country.

The methods of application in detail are described fully, as well as their actual application and results, in my work published in 1862 (2 vols.), entitled "The First Principles of Observational Seismology, as developed in the Report to the Royal Society of London of the Expedition made by Command of the Society into the Interior of the Kingdom of Naples, to investigate the Circumstances of the Great Earthquake of December, 1857," to the many illustrations of which the pecuniary grant, in aid, of ￡300 was most liberally made to the publishers (Messrs. Chapman and Hall) by the Society.

It is not my intention here, nor would space allow, of my going into the details of observation, nor of the deductions and conclusions I have recorded in those volumes. I have referred to their contents as marking the advent of a new method. I have ventured to call it a new organon in the investigation of Earthquakes, and, through them, of the deep interior of our earth; and will only add that the method, on this its very first trial, proved fertile and successful. The depth of focus for this shock of December, 1857, was about seven to eight geographical miles below sea level, roughly stated. It gives me great pleasure to add that my friend, Dr. Oldham, Director-General of the Geological Survey of India, has since applied these same methods to the phenomena of the great Cachar Earthquake of the 10th January, 1869, and with success. The pressure of official duties has, he informs me, as yet prevented his fully working out his results, but they appear so far to indicate, as we should expect, a depth of focus or origin considerably greater than in the European case of 1857. Some account of Dr. Oldham's results were this year communicated to the Geological Society of London through myself, they are of great interest and importance.

Such, briefly and imperfectly sketched, is the existing state of Seismology. As a branch of exact science it is, as it were, an affair of yesterday. It is with reluctance that I have been compelled, in this review, to refer to my own work so prominently. The harvest has been and still is plenteous, but in this field of intellectual work the labourers are few. This must continue to be so as long as Geology shall continue to be viewed in public estimation (in England at least) as a fashionable toy, that everyone who has been to school is supposed capable of handling; and until all who profess to be geologists shall have learnt that, to make sound progress, they must first become mathematicians, physicists and chemists.

It is to the general imperfect knowledge of these sciences amongst geologists that speculative errors show such vitality, and that Geology makes such poor progress towards becoming the interpretation of the world as a machine (Erdkunde).

It is for the same reason that Seismology and Vulcanology make little progress; the first cannot be pursued beyond its present boundaries, nor can even its present position be understood or explained by anyone unfamiliar with the laws of wave motion, of all classes of waves; and it would be easy to show, by quoting from various British or foreign text-books on Geology, how extremely imperfect is the grasp of some of the authors upon the subject of earthquake-wave motion, even such as they admit and endeavour to explain and apply: in fact, many geologists appear never to have framed to themselves any clear idea of what is a wave of any sort, liquid or elastic. The general silence as to seismic theory of French geological writers is remarkable, to whatever cause attributable. It has been said that French philosophers show themselves little disposed to acknowledge or to follow the lead of their foreign compeers in any branch of science. If this be true, or in so far as it may be so, it is unworthy of French science, which has such boundless claims upon our homage. I am disposed to attribute the fact in this case to other circumstances; and, amongst these, to the small extent to which our language is known amongst French scientific men.

Germany has shown more desire to cultivate this branch of science. Although, as yet, the distinct enunciation of its fundamental principles has but sparsely found its way into her text-books, several able monographs, such as those of Schmidt and of H?ttinger, prove how completely some of her philosophers have mastered and how well applied them. The men of science of Northern Italy, amongst whom so many glorious names are to be found on the roll of discovery, have shown themselves quite alive to the importance of Seismology; and I know of no more clear, exact and popular exposition of its principles and application, and of its cosmical relations, than is to be found in a small volume by Professor Gerolamo Boccardo, published at Genoa in 1869, entitled Sismopirologia Terremoti, Vulcani e lente oscillazione del suolo, saggio di una teoria di Geographia Fisica.

My object, so far, has been to mark the progress of ascertained theoretic notions as to Seismology. I have, therefore, passed without notice many speculative monographs, and the treatment upon Earthquakes, whether speculative or historical, and however able, that constitutes a prominent feature of nearly all systematic works on Geology.

That which may be at present viewed as achieved and certainly ascertained in theoretic Seismology is the clear conception of the nature of earthquake motion; the relations to it of great sea or other water wave commotions; the relations to it of sound waves-as to which, however, more remains to be known; and the relations of all these to secondary effects, tending in various ways to modify more or less the topographic and other conditions of the land or sea bottom. And in descriptive Seismology the present distribution of the earthquake bands or regions of greatest seismic prevalence and activity are tolerably ascertained, and their connection with volcanic lines and those of elevation rendered more evident. Viewed alone, nothing can yet be said to be absolutely ascertained as to the immediately antecedent cause or causes of the impulse. The function of Earthquake, as part of the cosmical machine, has become more clear, as the distinctive boundaries between Earthquake and permanent elevation of the earth have been made evident; and it has been seen that Earthquake, however contemporaneous occasionally with permanent elevation, is not the cause, though it may be one of the consequences of the same forces which produce elevation; and thus, that an infinite number of Earthquakes, however violent, and acting through however prolonged a time, can never act as an agent of permanent elevation, unless, indeed, on that minute scale in which surface elevation may arise from secondary effects, like that of the Ullah Bund.

Much remains to be done, and much may be expected even from the continuation, if done in a systematic and organised manner, of the statistic record of Earthquakes in connection with those other branches of cosmical statistics, Climatology, Meteorology, Terrestrial Magnetism, etc., the observation of which is already, to a certain extent, organised over a large portion of the globe.

And now let us look back for a moment to ask, How, by what mental path of discovery, have we arrived at what we have passed in review?

The facts of Earthquakes have been before men for unknown ages "open secrets," as Nature's facts have been well called; "but eyes had they and saw not." Facts viewed through the haze of superstition, or of foregone notions of what Nature ought to do, cease to be facts. When, after the great Calabrian Earthquake of 1783, the Royal Academy of Naples sent forth its commission of its learned members to examine into the effects, they had spread around them in sad profusion all that was necessary to have enabled them to arrive at a true notion of the nature of the shock, and thence a sound explanation of the varied and great secondary effects they witnessed, and of which they have left us the records in their Report, and the engravings illustrative of it. But we look in vain for any light; the things seen, often with distortion or exaggeration, are heaped together as in the phantasmagoria of a wild and terrible dream, from which neither order nor conclusion follow.

Why was this? Why were these eminent savants no more successful in explaining what they saw than the ignorant peasants they found in the Calabrian mountains?

Because physical science itself was not sufficiently advanced, no doubt; but also because they had no notion of applying such science as they had, to the very central point itself of the main problem before them, freed from all possible adventitious conditions, and so, as it were, attacking it in the rear. How different might have been the result of their labours, had they begun by asking themselves, What is an earthquake? Can we not try to find out what it is by observing and measuring what it has done? We see the converse mode of dealing with Nature in Torricelli. "Nature abhors a vacuum," was told him, as the wisdom of his day. Possibly: but her abhorrence is limited, for I find it is measured by the pressure of a column of water of thirty-four feet in height. We need not pursue the story with Pascal, up to the top of the Puy de D?me.

This lesson is instructive generally to all investigators, and particularly here; for Vulcanology, to which we are about now to turn, has occupied until almost to-day much the same position that Seismology did in those of the Neapolitan Commissioners.

Whole libraries have been written with respect to it dealing with quality, but measure and quantity remain to be applied to it.

To a very preponderant class in the civilised world no knowledge is of much interest or value that does not point to what is called a "practical result," one measurable into utility or coin. I do not stop to remark as to the bad or as to certain good results of this tendency of mind; but I may venture to point out to all, that the exact knowledge of the nature of earthquake motion, even during the short time that it has become known, has not been barren in results absolutely practical and utilitarian. The minute investigation of the destruction of buildings, etc., and the deductions that have been made as to the relations between the form, height, materials, methods of building, combination of timber and of masonry, and many other architectural or constructive conditions, have made it certain now that earthquake-proof houses and other edifices can be constructed with facility, and at no great increase, if any at all, of cost. I can affirm that there is no physical necessity why in frequently and violently shaken countries, such as Southern Italy or the Oriental end generally of the Mediterranean, victims should hereafter continue by thousands to be sacrificed by the fall of their ill-designed and badly built houses.

Were a "Building Act" properly framed, put in force by the Italian Government in the Basilicatas and Capitanata, etc., so that new houses or existing ones, when rebuilt, should be so in accordance with certain simple rules, a not very distant time can be foreseen when Earthquakes, passing through these rich and fertile but now frequently sorely afflicted regions, should come and go, having left but little trace of ruin or death behind. Some disasters there must always be, for we cannot make the flanks of mountains, nor the beds of torrents, etc., always secure; but the main mortality of all Earthquakes is in the houses or other inhabited buildings. Make these proof, and the wholesale slaughter is at an end.

The principles we have established have been thus practically applied in another direction. The Japanese Government, with the keen and rapid perception of the powers inherent in European science which characterises now that wonderful people, has commenced to illuminate its coasts by lighthouses constructed after the best European models. But Japan is greatly convulsed by earthquakes, and lighthouses, as being lofty buildings, are peculiarly liable to be destroyed by them.

The engineer of the Japanese Government for these lights, Mr. Thomas Stevenson, C.E. (one of the engineers to the Commissioners of Northern Lights), was instructed to have regard, in the design of those lighthouses, to their exposure to shock. I was consulted by Mr. Stevenson as to the general principles to be observed; and those edifices have been constructed so that they are presumedly proof against the most violent shocks likely to visit Japan; not, perhaps, upon the best possible plan, but upon such as is truly based upon the principles I have developed. Mr. Stevenson has published some account of their construction.

The earthquake regions of South America might with incalculable benefit apply those ideas; and, indeed, they have been, to some extent, already applied by my friend, Mr. William Lloyd, Member of the Institution of Civil Engineers, to the New Custom Houses constructed from his designs at Valparaiso.

As one of these utilitarian views, and an important one, it will occur to many to ask-Can the moment of the occurrence or the degree of intensity of earthquake shock be predicted, or is it probable that at a future day we may be able to predict them? At present, any prediction, either of the one or the other, is impossible; and those few who have professed themselves in possession of sufficient grounds for such prediction are deceivers or deceived. Nor is it likely that, for very many years to come, if ever, science shall have advanced so as to render any such prediction possible; but it is neither impossible nor improbable that the time shall arrive when, within certain, perhaps wide, limits as to space, previous time, and instant of occurrence, such forewarnings may be obtainable.

Earthquakes, like storms and tempests, and nearly all changes of weather, are not periodic phenomena, nor yet absolutely uncertain or, so to say, accidental as to recurrence.

They are quasi-periodic, that is to say, some of their conditions as to causation rest upon a really periodic basis, as, for example, the recurrence of storms upon the periodic march of the earth, and sun and moon, etc., and the recurrence of Earthquakes upon the secular cooling of our earth; but the conditions in both are so numerous and complicated with particulars, that we cannot fully analyse them-hence, cannot reduce the phenomena to law, and so cannot predict recurrence. Yet storms and tempests-which were, along with pestilences and Earthquakes, amongst the natural phenomena which Bishop Butler deemed in his own day impossible of human prediction-have already, through the persistent and systematised efforts of meteorological observers, become to a certain extent foreseeable; and medical science assures us that it has rendered that, though to a much less degree of probability, true of pestilences.

We may, therefore, give the utilitarian some hope, that if he will help us along-who value our accessions of knowledge primarily upon a different standard to his-in our talk of discovery, our posterity, in a century or two hence, may not improbably possess the advantage of being able, in some degree, to predict their Earthquakes. I fear the inducement will go but a small way with the utilitarian generation, whose bent tends much towards asking, "What has posterity ever done for them?"

But though we cannot as yet predict the time when an Earthquake may take place in any locality, we can, on mixed statistic and dynamic grounds, in many cases state the limits of probable violence of the next that may recur. For example, the three shafts of marble columns of the Temple of Serapis, at Pozzuoli, each of about 411/2 feet in height, and 4 feet 10 inches in diameter at the base, remain standing alone, since they were uncovered, in the year 1750.

Now, as we can calculate exactly what velocity of earthquake-wave motion would be required to overset these, we are certain that, during the last one hundred and twenty-two years, the site of the Temple, and we may say Naples and the Phlegr?an fields generally, have never experienced a shock as great as the very moderate one that would overset these columns. A shock whose wave particle had a horizontal velocity of only about 31/2 feet (British) per second would overturn these columns; which is only about one-fourth the velocity (within the meizoseismic area) of the great shock of 1857, that produced wide-spread destruction in the Basilicatas, and not enough to throw down any reasonably well-built house of moderate height.

Naples, so far as Earthquake is concerned, whether coming from the throes of Vesuvius or elsewhere, has a pretty good chance of safety. She may possibly (though not probably) be some day smothered in ashes; but is in little danger of being shaken to the earth. During this time there have been taking place, larger eruptions of Vesuvius and earthquake shocks from other centres, together probably about the same number of times as the numbers of those years, when those columns have been more or less shaken.

We may therefore affirm that the probability (on the basis of this experience only) is, say 120 to 1, that the next shock, whether derived from Vesuvius, or elsewhere, that may shake Pozzuoli, will be one less in power than would be needed to overturn the shafts of the Temple of Serapis there.

* * *

Let us now turn to the second branch of our subject-viz., Vulcanology-upon which, as yet, we have secured less firm standing ground than we have seen we possess in Seismology, for which reason we took that first into consideration.

It is the part of Vulcanology to co-ordinate and explain all the phenomena of past or present times visible on our globe which are evidences of the existence and action, whether local or general, of temperatures within our globe greatly in excess of those of the surface, and which reach the fusing points of various mineral compounds as found arriving, heated or fused, at the surface.

The stratigraphic geologist sees that such heated or fused masses have come up from beneath, throughout every epoch that he can trace; but he cannot fail to discern more or less a change in the order or character of those outcomings, as he traces them from the lowest and oldest formations to those of the present day. He sees immense outpourings of granitoid or porphyrytic rocks that have welled up and overflowed the oldest strata-huge dykes filling miles of fissures that had been previously opened for the reception of the molten matter that has filled them, and often passing through those masses of previously outpoured rock; later he sees huge tables of basaltic rock poured forth over all. One grand characteristic common to all these-commonly called plutonic products-being that, whether they were poured forth over the surface or injected into cavities in other rocks, the movements of the fused material were, on the whole, hydrostatic and not explosive.

At the present day, whatever other evidences we have of high temperature below our globe's surface, that which primarily fixes the eye of the geologist is the Volcano, whose characteristic, as we see it in activity, is explosive. But though there is this great characteristic difference between the plutonic and the volcanic actions and their products, the two, when looked at largely, are seen so to inosculate, that it is impossible not to refer them to an agency common to both, however changed the modes of its action have been between the earliest epochs of which traces are presented to us and the present day.

To us little men, who, as Herschell has well said, in referring to the methods of measuring the size of our globe, "can never see it all at once, but must creep like mites about its surface," the Volcano, in the stupendous grandeur of its effects, tends to fix itself in our minds in exaggerated proportions to its true place in the cosmic machine; and, in fact, nearly all who have sought to expound its nature and mode of origination have occupied themselves far too exclusively with describing and theorising upon the strange and varied phenomena which the volcanic cone itself and its eruptions present, and too often, in the splendour and variety of these, have very much lost sight of what ought to be the centre-point of all such studies, namely, to arrive at some sound knowledge of what is the primum mobile of all these wonderful efforts. Nor has the distinction been very clearly seen between the main phenomena presented at and about volcanic active mouths, which can be employed to elucidate the nature of the causation at work far below, and those most varied and curious, and in other respects most pregnant and instructive phenomena, mechanical and chemical, which are called into action in and by the ejected matter of the volcanic cone after its ejection. It can help us but little or very indirectly, in getting at a true conception of the nature and source of the heat itself of the Volcano, to examine, for example, all the curious circumstances that are seen in the movements and changes in the lava that has already flowed from its mouth; but it would be of great importance if we can ascertain, by any form of observation around the cone, from what depth it has come, or at what depth the igneous origin lies.

The physician, endeavouring to ascertain the real nature of small-pox or measles, will scarcely make much progress who, however curiously or minutely, confines his attention to the pustules that he sees upon the skin.

Yet the Volcano, or rather all volcanic activity as now operative upon our globe, is, as it were, an experiment of Nature's own perpetually going on before us, the results of which, if well chosen-that is, as Bacon says, by keeping to the main and neglecting the accidents-can, when colligated and correctly reasoned upon, in relation to our planet as a whole, give us the key to the enigma of terrestrial Vulcanicity in its most general sense, and at every epoch of our world's geognostic history, and show us its true place and use in the cosmical machine. Let us glance at the history of past speculation on this subject, from which so little real knowledge is to be derived, and then at the salient facts of Vulcanology as now seen upon our earth, and finally see if we can connect these with other great cosmical conditions, so as to arrive at a consistent explanation in harmony with all.

We gain nothing absolutely from the knowledge of the so-called "ancients" as to Volcanoes in Europe at least, where alone historic records likely to refer to them exist. The Volcanoes of Europe are few and widely scattered. The Greeks saw but little of them, and the Romans were all and at all times most singularly unobservant of natural phenomena.

C?sar never mentions the existence in France of the Volcanoes of Auvergne, so much like those he must have seen in Italy and Sicily; and Roman writers pass in silence that great volcanic region, though inhabited by them, and their language impressed upon the places, as Volvic (volcano-vicus) seems with others to indicate; and though there is some reason to believe that one or other of the Puys was in activity within the first five hundred years of our epoch, the notices which Humboldt and others have collected as from Plato, Pausanius, Pliny, Ovid, etc., teach nothing.

Whatever of mere speculation there may have been, volcanic theory, or what has passed for such, there was none before 1700, when Léméry brought forward a trivial experiment, the acceptance of which, even for a moment, as a sufficient cause for volcanic heat (and it retarded other or truer views for years), we can now only wonder at. Breislak's origin, in the burning of subterranean petroleum or like combustibles, was scarcely less absurd than Léméry's sulphur and iron filings.

Davy, in the plenitude of his fame, and full of the intense chemical activities of the metals of the alkalies which he had just isolated, threw a new but transient verisimilitude upon the so-called chemical theory of Volcanoes, by ascribing the source of heat to the oxidation of those metals assumed to exist in vast, unproved and unindicated masses in the interior of the earth. But Davy had too clear an intellect not to see the baseless nature of his own hypothesis, which in his last work, the "Consolations in Travel," he formally recanted; and it only survived him in the long-continued though unconvincing advocacy of Dr. Daubeny. So far, the origin of the heat had been sought always, in the crude notion of some sort of fuel consumed, whether that were petroleum or potassium and sodium; but as no fuel was to be found, nor any indicated by the products, so far as known, of the volcanic heat, so what has been called the mechanical theory, in a variety of shapes, took its place.

This, in whatever form, takes its lava and other heated products of the volcano ready made from a universal ocean of liquid material, which it supposes constitutes the interior or nucleus of our globe, and which is only skinned over by a thin, solid crust of cooled and consolidated rock, which was variably estimated at from fourteen to perhaps fifty miles in thickness. Here was a boundless supply of more than heat, of hot lava ready made, the existence of which at these moderate depths the then state of knowledge of hypogeal temperature, which was supposed to go on increasing with depth at the rate of about 1° Fahrenheit, for every thirty or forty feet, seemed quite to sustain.

The difficulty remained, how was this fiery ocean brought to the surface or far above it? To account for this two main notions prevailed, and, indeed, have not ceased to prevail. Some unknown elastic gases or vapour forced it up through fissures or rents pre-existent, or produced by the tension of the elastic and liquid pressure below.

The form in which this view took most consistency, and approaching most nearly to truth, finds the elastic vapour in steam generated from water passed down through fissures from the sea or from the land surface. But to this the difficulty was started, that fissures that could let down water would pass up steam. The objection, when all the conditions are adequately considered, has really no weight; and it has been completely disposed of, since within a few years it has been proved that capillary infiltration goes on in all porous rocks to enormous depths, and that the capillary passages in such media, though giving free vent to water-and the more as the water is warmer-are, when once filled with liquid, proof against the return through them of gases or vapours. So that the deeply seated walls of the ducts leading to the crater, if of such material, may be red hot and yet continue to pass water from every pore (like the walls of a well in chalk), which is flushed off into steam that cannot return by the way the water came down, and must reach the surface again, if at all, by the duct and crater, overcoming in its way whatever obstructions they may be filled with.

And this remarkable property of capillarity sufficiently shows how the lava-fused below or even at or above the level of infiltration-may become interpenetrated throughout its mass by steam bubbles, as it usually but not invariably is found to be.

Nor is it difficult to see such a mechanism between volcanic ducts and fissures conveying down water, as large and open pipes, for a large part of their depth, as shall bring down water to foci of volcanic heat, without the power of the water flowing back except as steam and through the crater.

Indeed, the facts known as to geysers, and those of half-drowned-out Volcanoes such as Stromboli-whose action is intermittent just as much as that of a geyser-show that this is not merely probable. There is, therefore, no need for the hypothesis of those who have supposed all the huge volumes of steam blown off from Volcanoes in eruption to come from vesicular water pre-existent in the minute cavities of crystalline or other rocks before their fusion into lava: a fact not proved for many classes of rock, and for none in sufficient quantity to account for the vast volume of steam required and for the irregularity of its issue.

It is rather to anticipate, but I may state at once that, so far as the admission of superficial waters to the interior, and to any depth to which fissures or dislocation can extend, I believe no valid physical or mechanical difficulties exist, taking into account all the conditions that may come into play together.

Another set of views has been suggested and supported by various writers, which proposes to account for the rise of lava on purely hydrostatic principles. The solid crust, fractured into isolated fragments by tensions due to its own contraction, is supposed to sink into the sea of lava on which it floats; and much ingenuity has been expended in imagining the mechanism by which, in places, the liquid matter is supposed to rise above the surface of the crust.

I have no space for discussing these views further than to assert that, in the existing state of our globe, and even admitting a solid crust of only 60,000 metres thick, dislocation of the crust by tension is not possible. The solid crust of our globe, as I hope we shall see further on, is not in a state of tension, and has not been so since it was extremely thin, a mere pellicle as compared with the liquid nucleus, but is, on the contrary, in a state of tangential compression.

However tenable, in other respects, may be the volcanic theory which rests upon the assumption of a very thin crust and a universal ocean of fused rock beneath, it fails wholly to explain many of the most important circumstances observable as to the distribution and movements of existing Volcanoes on our globe.

It affords no adequate explanation of the configuration of the lines of Volcanoes, nor of their occurrence in the ocean bed, nor of their existence in high latitudes, near the Poles, where, no matter how or at what rate our globe cooled from liquidity, the crust must be thickest; nor of the independence of eruptive action of closely adjacent volcanic vents; nor of the non-periodicity, the sudden awakening-up to activity, the as sudden exhaustion, the long repose, the gradual decay of action at particular vents, and of much more that might be stated and sustained as difficulties left by that theory unexplained, or that are of a nature even opposed to it.

The researches of the last few years have, however, as it appears to me, rendered any theory that demands as its postulates a very thin crust, and a universal liquid nucleus beneath it, absolutely untenable.

Without attaching any importance to the arguments of Mr. Hopkins, based upon precession and nutation, it appears to me, on various other grounds, some of which have been urged by Sir William Thompson, that the earth's solid crust is not a thin one, at least not thin enough to render it conceivable that water can ever gain admission to a fluid nucleus, if any such still exist, situated at so great a depth; and without such access we can have no Volcano. It is not necessary to go to the extent of a crust of 800 or 1,000 miles thick: with one of half the minor thickness, I believe it may be proved, on various grounds, hydraulic amongst others, that neither water could reach the nucleus, nor the liquid matter of the nucleus reach the surface. Mr. Hopkins having proved to his own satisfaction an enormous thickness for the crust, and seeing clearly the difficulties that this involved to the generally accepted volcanic theory, and having no other to substitute for it, fell back upon that most vague and weak notion of the existence of isolated lakes of liquid rock, existing at comparatively small depths beneath the earth's surface within the solid and relatively cold crust, each supplying its own Volcano, or more than one, with ready-made lava. What is to produce these lakes of fused matter in the midst of similar solidified matter? what is perpetually to maintain their fluidity in the midst of solid matter continually cooling? what has given them their local position? why near or less near the surface? what should have arranged them in directions stretching in some cases nearly from Pole to Pole?

Surely this creation of imaginary lakes, merely because it happens to fit the vacant chink that seems needed to wedge up a falling theory, is an instance of that abuse of hypothesis against which Newton so vehemently declaims-"Hypotheses non fingo."

Hypothesis, to be a philosophic scaffolding to knowledge, must, as Whewell has said, "be close to the facts, and not merely connected with them by arbitrary and untried facts." Yet this appears accepted by Lyell (10th edition, Vol. II., p. 227, and elsewhere); by Phillips ("Vesuvius," pp. 331, 332); by Scrope, if, as I hope, I mistake him not ("Volcanoes," pp. 265, 307-8); though none of these excellent authorities seem either quite clear or quite satisfied with the notion; and in the very passage referred to, Lyell may have possibly a much more philosophic notion in view, where he says: "It is only necessary, in order to explain the action of Volcanoes, to discover some cause which is capable of bringing about such a concentration of heat as may melt one after the other certain portions of the solid crust, so as to form seas, lakes or oceans of subterraneous lava." (Vol. II, pp. 226, 227). If by this is meant, that all that is needed to complete a true theory of volcanic action is to discover an adequate cosmical cause for the heat-that is to say, a prime mover to which all its phenomena may be traced back, which shall be at once reconcilable with the conditions of our planet as a cooling mass in space and with facts of Vulcanology as they are now seen upon it-then I entirely agree with it.

It has been my own object to endeavour to discover and develope that adequate cause in a Paper "On Volcanic Energy, an Attempt to develope its True Nature and Cosmical Relations," read (in abstract) before the Royal Society of London ("Proceedings, Royal Society," Vol. XX., May, 1872), and now (October, 1872) under consideration of Council with a view to publication.

I propose concluding this review of the progress of Vulcanology (in which I have had to limit myself to reviewing merely the chief stages of advance towards knowledge of the nature and origin of volcanic heat itself, and have had to pass without notice the vast and important mass of facts and reasonings collected by so many labourers as to its visible phenomena and products, and the still greater mass of speculation, good and bad, on every branch of the subject), by giving a necessarily very brief and imperfect sketch of my own views as in that Paper in part developed. It will first be necessary to retrace our steps a little, in order to gain such a point as shall afford us a fuller view of the whole problem before us.

It is not necessary to dilate, even did space allow, upon the many points which bind together Earthquakes and Volcanoes as belonging to the play of like forces. These are generally admitted; and in various ways, more or less obscure, geologists generally have supposed some relations between these and the forces of elevation, which have raised up mountain chains, etc.

No one, however, that I am aware of, prior to myself, in the Paper just alluded to, has attempted to show, still less to prove upon an experimental basis, that all the phenomena of elevation, of volcanic action, and of Earthquakes, are explicable as parts of one simple machinery-namely, the play of forces resulting from the secular cooling of our globe. We have seen that, on the whole, both Earthquakes and Volcanoes follow along the great lines of elevation of our surface. Any true solution of the play of forces which has produced any one of those three classes of phenomena must connect itself with them all, and be adequate to account for all. And this would have earlier been seen, had geologists generally framed for themselves any correct notions of the mechanism of elevation itself, and seen its real relation with the secular cooling of our planet. But the play of forces resulting from this secular cooling has never, until very recently, been adequately or truly stated. The arbitrary assumption and neglect of several essential conditions by La Place, in his celebrated Paper "On the Cooling of the Earth," in the fifth volume of the "Mécanique Céleste," and the arbitrary and unsustainable hypothesis of Poisson upon the same subject, have tended to retard the progress of physical Geology as to the nature of elevation: the first, by leaving the geologist in doubt as to whether our globe were cooling at all; the second, by suggesting distorted notions as to the mode of its cooling and consolidation. On the other hand, neither geologists nor mathematicians generally have framed for themselves any clear notions of the mechanism of elevation. Had a true conception been formed of the forces and interior movements brought necessarily into operation by the secular cooling of the globe, geologists could scarcely have failed to see that their notion as to the way and direction in which the forces producing elevation have actually acted could not, if arising from refrigeration, be those which they have almost universally supposed, namely-some force acting vertically upwards, i.e., radially from the centre of the sphere. Had geologists only looked at Nature with open eye, they must have seen that mountain ranges, and elevations generally (exclusive of volcanic cones), presented circumstances absolutely incompatible with their having been thrust up by any force primarily acting in the direction of a radius to the spheroid.

Yet this is the erroneous notion of the mechanism of elevation which to the present hour prevails amongst geologists, so far as they in general have framed to themselves any distinct idea of such mechanism at all.

Thus, only to cite two examples from recent authors of justly high reputation. Lyell says of the probable subterranean sources, whether of upward or downward movement, when permanently uplifting a country, and in reference to the crumpling of strata on mountain flanks by lateral pressure, it would be rash to assume these able to resist a power of such stupendous energy, "if its direction, instead of being vertical, happened to be oblique or horizontal." This is somewhat vague-and I trust I do not mistake or misrepresent the illustrious author-yet it is the most explicit expression I can find in the "Principles of Geology" as to his notion of the primary direction of elevatory force (Edit. 10, Vol. I., p. 133). That Mr. Scrope's idea is that only of primary radial or vertical direction of such forces, is apparent on inspecting his Diagram No. 64 ("Volcanoes," p. 285), and in the use of the words, "an axial wedge of granite," which, on the next page, we find is "liquefied granite;" and if we read on to page 294, and refer also to pages 50 and 51, I believe there can be no doubt that vertical or direct up-thrust is the author's notion of the primary direction of all forces of elevation. The true nature of these forces was, however, clearly seen and most justly stated by Constant Prevost ("Compt. Rend.," Tome XXXI., 1850, and "Bulletin de la Société Géolog. de France," Tome II., 1840) as consisting, not in forces of some unknown origin acting primarily in the vertical, but in tangential pressures acting horizontally, and resolved by mutual pressures at certain points into vertical resultants. These Prevost rightly attributed to the contraction of the earth's solid crust. The same idea has been adopted by Elie de Beaumont as the true mechanism of the elevation of mountain ranges; and although De Beaumont's views as to the thinness he assigns to the solid and contracting crust, and his strange deduction as to the parallelism of contemporaneous mountain chains uplifted by its spasmodic action along certain lines, may be untenable, his notion generally as to the play of forces producing mountain elevation is much more nearly correct.

Mr. Hopkins's notion is simply that of the geologists. Anyone who reads his well-known papers on elevation and the formation of fissures, etc., must see that he views all elevatory forces as of liquids or quasi-liquids forced up and acting primarily vertically upon the strata above them, and that these strata are not under tangential compression, but under tension. Hence the mathematical deductions contained in those papers as to the directions in which elevatory forces act, and in which fissures are formed by them, are not in any way a setting forth of such facts as occur in Nature, and, much attention as they have attracted, can only now be viewed as exercises of mathematical skill misapplied, because based upon data not to be found in Nature. In fact, those papers do but misrepresent Nature, and, like many other mathematical investigations based on untrue or insufficient data, have tended to retard knowledge.

The views which I have put forward in the Paper I have referred to, read to the Royal Society, recapitulated in skeleton, so to say, are as follows. Omitting those portions which treat of our globe from the period of the first liquefaction out of a nebulous condition, and of the earliest stages of the cooling by radiation into space, when the crust was extremely thin, and of the deformation of the spheroid as one of the first effects of its contraction, and through that the general shaping out of continents and ocean beds; I have endeavoured to show that the rate of contraction of the crust, while very thin, exceeded that of the large fluid nucleus supporting it, and so gave rise to tangential tensions in the crust, and fracturing it into segments; next, that as the crust thickened, these tensions were gradually converted into tangential pressures, the contraction of the nucleus now beginning to exceed (for equal losses of heat) that of the crust through which it cooled. At this stage these tangential pressures gave rise to the chief elevations of mountain chains-not by liquid matter by any process being injected from beneath vertically, but by such pressures, mutually reacting along certain lines, being resolved into the vertical, and forcing upwards more or less of the crust itself. The great outlines of the mountain ranges and the greater elevation of the land were designated and formed during the long periods that elapsed in which the continually increasing thickness of crust remained such that it was still, as a whole, flexible enough, or opposed sufficiently little resistance to crushing, to admit of this uprise of mountain chains by resolved tangential pressures. I have shown that the simple mechanism of such tangential pressures is competent to account for all the complex phenomena both of the elevations and of the depressions that we now see on the earth's surface (other than continents and ocean beds), including the production of gaping fissures (in directions generally orthogonal to those of tangential pressure). And as our earth is still a cooling body, and the crust, however now thicker and more rigid, is still incapable of sustaining the tangential pressures to which it is now exposed, so I by no means infer that slow and small (relatively) movements of elevation and depression may not be still and now going on upon the earth's surface; in fact all the phenomena of elevation and depression, rending, etc., which at a much remoter epoch acted upon a much grander and more effective scale. So that, for aught my views say to the contrary, all the mountain chains in the world may be possibly increasing in stature year by year, or at times; but in any case at a rate almost infinitesimally small in its totality over the whole earth to that with which their ridges were originally upreared.

But the thickness of the earth's crust-thus constantly added to, by accretion of solidifying matter from the still liquid or pasty nucleus, as the whole mass has cooled-has now assumed such a thickness as to be able to offer a too considerable resistance to the tangential pressures, to admit of its giving way to any large extent by resolution upwards; yet the cooling of the whole mass is going on, and contraction, though unequal, both of thick crust and of hotter nucleus beneath also, whether the latter be now liquid or not. Were the contraction, lineal or cubical, for equal decrements or losses of heat, or in equal times-equal both in the material of the solidified crust and in that of the hotter nucleus-there could be no such tangential pressures as are here referred to, at any epoch of the earth's cooling. But in accordance with the facts of experimental physics, we know that the co-efficient of contraction for all bodies is greater as their actual temperature is higher, and this both in their solid and liquid states.

Hence for equal decrements of heat, or by the cooling in equal times, the hotter nucleus contracts more than does its envelope of solid matter.

The result is now, as at all periods since the signs changed of the tangential forces thus brought into play-i.e., since they became tangential pressures-that the nucleus tends to shrink away as it were from beneath the crust, and to leave the latter, unsupported or but partially supported, as a spheroidal dome above it.

Now what happens? If the hollow spheroidal shell were strong enough to sustain, as a spheric dome, the tangential thrust of its own weight and the attraction of the nucleus, the shell would be left behind altogether by the nucleus, and the latter might be conceived as an independent globe revolving, centrally or excentrically, within a shell outside of it. This, however, is not what happens.

The question then arises, Can the solid shell support the tangential thrust to which it would be thus exposed? By the application to this problem of an elegant theorem of Lagrange, I have proved that it cannot possibly do so, no matter what may be its thickness nor what its material, even were we to assume the latter not merely of the hardest and most resistant rocks we know anything of, but even were it of tempered cast-steel, the most resistant substance (unless possibly iridio-osmium exceed it) that we know anything about. Lagrange has shown that if P be the normal pressure upon any flexible plate curved in both directions, the radii of these principal curvatures being ρ' and ρ'', and T the tangential thrust at the point of application and due to the force P, then:

P = T (1/ρ' + 1/ρ'')

When the surface is spherical, or may be viewed as such, ρ' = ρ'' and

P = 2T / ρ or, T = P × ρ/2

In the present case P is for a unit square (taken relatively small and so assumed as plane) of the shell, suppose a square mile, equal to the effect of gravity upon that unit, ρ being the earth's radius, and if we assume the unit square be also a unit in thickness, P is then the weight of a cubic mile of its material; and if we take (roughly) the earth's radius as 4,000 miles, the tangential pressure, T, is, on each face of the cubic mile, equal to

(4000/2) P,

or equal to the pressure of a column of the same material of 2,000 times its weight.

If the cubic mile that we have thus supposed cut out of the earth's crust at the surface were of the hardest known granite or porphyry, it would be exposed to a crushing tangential pressure equal to between 400 and 500 times what it could withstand, and so must crush, even though only left unsupported by the nucleus beneath, to the extent of 1/400 or 1/500 of its entire weight. And what is true here of a mile taken at the surface, is true (neglecting some minute corrections for difference in the co-efficient of gravity, etc.) if taken at any other depth within the thick crust.[F]

The crust of our earth, then, as it now is, must crush, to follow down after the shrinking nucleus-if so be that the globe be still cooling, and constituted as it is; even to the limited extent to which we know anything of its nature-it must crush unequally, both regarded superficially and as to depth; generally the crushing lines being confined to the planes or places of greatest weakness; and the crushing will not be absolutely constant and uniform anywhere, or at any time, or at any of those places of weakness to which it will be principally confined, but will be more or less irregular, quasi-periodic, or paroxysmal: as is, indeed, the way in which all known material substances (more or less rigid) give way to a slow and constantly increasing, steady pressure.

We have now to ask, How much of this crushing is going on at present year by year? And the answer to this depends upon what amount of heat our world is losing into space year by year.

Geologists who have taken on trust the statement, that La Place has proved that the world has lost no sensible amount of heat for the last 10,000 years seem generally to suppose that to be a fact; but in reality La Place has proved nothing of the sort, as those geological teachers who have echoed the conclusion should have known, had they deciphered the mathematical argument upon which it has been supposed to rest.

By application of Fourier's theorem (or definition) to the observed rate of increment of heat in descending from the geothermal couche of invariable temperature, and the co-efficients of conductivity of the rocks of our earth's crust, as given by the long-continued observations made beneath the Observatories of Paris and of Edinburgh, it results that the annual loss of heat into space of our globe at present is equal to that which would liquefy into water, at 32° Fahr., about 777 cubic miles of ice; and this is the measuring unit for the amount of contraction of our globe now going on. The figures are not probably exact, for the data are not on a basis sufficiently full or exactly established as yet; but they are not very widely wrong, and their precise exactness is not material here. Now, how is this annual loss of heat (great or small, as we may please to view it) from the interior of our globe disposed of?

What does it do in the interior? We have already seen that it is primarily disposed of by conversion into work; into the work of diminishing the earth's volume as a whole, and in so doing crushing portions of the solid surrounding shell.

But does the transformation of lost heat into the work of vertical descent, and of the crush as it follows down after the shrinking nucleus, end the cycle? No. A very large portion of the mechanical work thus produced, and resolved, as we have seen, into tangential crushing pressure, is retransformed into heat again in the very act of crushing the solid material of the shell. If we see a cartload of granite paving-stones shot out in the dark, we see fire and light produced by their collision; if we rub two pieces of quartz together, and crush thus their surfaces against each other, we find we heat the pieces and evolve light.

The machinery used for crushing by steam-power, hard rocks into road metal, gets so hot that the surfaces cannot be touched.

These are familiar instances of one result of what is now taking place by the crushing of the rocky masses of our cooling and descending earth's crust, every hour beneath our feet, only upon a vastly greater scale. It is in this local transformation of work into heat that I find the true origin of volcanic heat within our globe. But if we are to test this, so as in the only way possible to decide is it a true solution of this great problem, we must again ask the question, How much? and to answer this, we must determine experimentally how much heat can be developed by the crushing of a given volume, say a cubic mile, of such rocky materials as we know must constitute the crust of our globe down to the bottom of the known sedimentary strata, and extending to such crystalloid rocks as we may presume underlie these. We must also obtain at least approximately what are the co-efficients of total contraction between fusion and atmospheric temperature of such melted rocks, basic and acid silicates, as may be deemed representative of that co-efficient for the range of volcanic fused products, basalts, trachytes, etc., which probably sufficiently nearly coincide with that of the whole non-metallic mass of our globe.

The first I have determined experimentally by two different methods, but principally by the direct one of the work expended in crushing prisms of sixteen representative classes of rock; the specific gravities and specific heats of which I have also determined.

If H be the height of a prism of rock crushed to powder by a pressure, P, applied to two opposite faces, which, when the prism has been reduced to its volume in powder, has acted through a range of H - t, then

P × (H - t) / 772

is the heat corresponding to the work expended in the crushing, expressed in British units of heat. The following were the rocks experimented upon: Caen stone, Portland (both oolites), magnesian limestone, sandstones of various sorts, carboniferous limestones (marbles), the older slates (Cambrian and Silurian), basalts, various granites and porphyries, thus ranging from the newest and least resistant to the oldest and most resistant rocks. The results have been tabulated, and are given in detail in my Paper, now in possession of the Royal Society. The minimum obtained is 331 and the maximum 7,867 British units of heat developed, by transformation of the work of crushing one cubic foot of rock. If we apply the results to a thickness of solid crust of 100 miles (British), of which the upper twenty-one miles consist of neozoic, newer pal?ozoic, older pal?ozoic and azoic rocks in nearly equal proportion as to thickness, and the remaining eighty miles of crystalloid rocks (acid and basic magmas of Durocher) of physical properties which we may assume not very different from those of our known granites and porphyries-and which, in so far as they may differ, would give a still higher co-efficient of work transformed into heat than I have attributed to them by ranging them as only equal to the granites, etc.-then we obtain a mean co-efficient for the entire thickness of crust of 100 miles of 6,472 British units of heat, developable from each cubic foot of its material, if crushed to powder. It results from this that each cubic mile of the mean material of such a crust, when crushed to powder, developes sufficient heat to melt 0·876 cubic miles of ice into water at 32°, or to raise 7·600 cubic miles of water from 32° to 212° Fahr., or to boil off 1·124 cubic miles of water at 32° into steam of one atmosphere, or, taking the average melting point of rocky mixtures at 2,000° Fahr., to melt nearly three and a-half cubic miles of such rock, if of the same specific heat.

Of the heat annually lost by our globe and dissipated into space, represented by 777 cubic miles of ice melted, as before stated, the chief part is derived from the actual hypogeal source of a hotter though not necessarily fused nucleus, and nearly, if not wholly, is quite independent of the heat of Vulcanicity, which is developed as a consequence of its loss or dissipation. But were we to take the extreme case, and suppose it possible that all the heat the globe loses annually resulted from the transformation of the work of internal crushing of its shell, we shall find that the total volume of rock needed to be crushed in order to produce the required amount of lost heat is perfectly insignificant as compared with the volume of the globe itself, or that of its shell. For, as 1·270 cubic miles of crushed rock developes heat equivalent to that required to melt one cubic mile of ice to water at 32°, and if we assume the volume of our globe's solid crust to equal one-fourth of the total volume of the entire globe, 987 cubic miles of rock crushed annually would supply the whole of the heat dissipated in that time. But that is less than the one sixty-five millionth of the volume of the crust only.

But a very small portion of the total heat annually lost by our globe is sufficient to account for the whole of the volcanic energy of every sort, including thermal waters, manifested annually upon our earth. In the absence of complete data, we can only approximately calculate what is the annual amount of present volcanic energy of our planet. This energy shows itself to us in three ways: 1. The heating or fusing of the ejected solid matters at volcanic vents. 2. The evolution of steam and other heated elastic fluids by which these are carried. 3. The work of raising through a certain height all the materials ejected. To which we must add a large allowance for waste, or thermal mechanical and chemical energy ineffectually dissipated in and above the vents. All these are measurable into units of heat.

I have applied this method of calculation to test the adequacy of the source I have assigned for volcanic heat, in two ways, viz.: 1. To the phenomena presented during the last two thousand years by Vesuvius, the best known Volcano in the world; and 2. To the whole of the four hundred and odd volcanic cones observed so far upon our globe, of which not more than one-half have ever been known in activity.

It is impossible here to refer to the details of the method or steps of these calculations. The result however is, that making large allowances for presumably defective data, less than one-fourth of the total telluric heat annually dissipated (as already stated in amount) is sufficient to account for the annual volcanic energy at present expended by our globe.

It is thus represented by the transformation into heat of the work of crushing about 247 cubic miles of (mean) rock, a quantity so perfectly insignificant, as compared with the volume of the globe itself, as to be absolutely inappreciable in any way but by calculation; and as its mechanical result is only the vertical transposition transitorily of material within or upon our globe, the proportion of the mass of which to the whole is equally insignificant, so not likely in any way to produce changes recognisable by the astronomer.

Space here forbids my entering at all upon that branch of my investigation which is based upon the experimental results, above mentioned, of the total contraction of fused rocks: for these, the original Paper can, I hope, be hereafter referred to. I am enabled, however, to prove thus how enormously more than needful has been the store of energy dissipated since our globe was wholly a melted mass, for the production, through the contraction of its volume, of all the phenomena of elevation and of Vulcanicity which its surface presents. And how very small is the amount of that energy in a unit of time as now operative, when compared with the same at very remote epochs in our planet's history.

I have said that if we can find a true cause in Nature for the origination of volcanic heat, all the other known phenomena, at and about volcanic vents, become simple. Lavas and all other solid ejecta of Volcanoes, from all parts of the earth's surface, as well as basalts, present in chemical and physical constitution close resemblance, and may be all referred to the melting of more or less fusible mixtures of siliceous crystalloid rocks with aluminous (slates, etc.) and calcareous rocks. Their general chemical composition, and the higher or lower temperatures of fusion resulting therefrom, together with the higher or lower temperatures to which they have been submitted at the different volcanic foci, determine their difference of flow (under like surface conditions) and of mineral character after ejection and cooling.

St. Clair de Ville and Fouqué have shown that the gaseous ejections, of which steam forms probably 99 per cent., are such as arise from water admitted to a pre-existent focus of high temperature.

Whether sea or fresh water is not material, when we bear in mind that the chemical constituents found in sea water and in natural fresh waters that have penetrated the soil are, on the whole, alike in kind and only differ in proportions. But I must pass almost without notice all the varied and instructive phenomena which are presented by volcanic vents, for to treat of these at all would be to more than double the size of this sketch.

In the source that has been pointed out as that from which volcanic heat itself is derived, viz., the secular cooling of our globe, and the effects of that upon its solid shell, we are enabled to point to that which is the surest test of the truth of any theory-that it not only enables us to account for all the phenomena, near or remote, but to predict them. We see here linked together as parts of one grand play of forces, those of contraction by cooling, producing by direct mechanical action the elevation of mountain chains, and by their indirect action, by transformation of mechanical work into heat, the production of Volcanoes; and both by direct and by indirect action, of Earthquakes, never previously shown to have thus the physical connection of one common cause, but merely supposed, more or less, to be connected by their distribution upon our earth's surface.

We now discern thus the physical cause why Volcanoes are distributed, viewed largely, linearly, and follow the lines of elevation; we see equally why their action is uncertain, non-periodic, fluctuating in intensity, with longer or shorter periods of repose, shifting in position, becoming extinct here, appearing in new activity or for the first time there. We have an adequate solution of the before inexplicable fact of their propinquity, and yet want of connection. We have an adequate cause for the fusion of rock at local points without resorting to the baseless hypothesis of perennial lakes of lava, etc.

For the first time, too, we discern a true physical cause for earthquake movement, where volcanic energy does not show itself. The crushing of the world's solid shell, whether thick or thin, goes on per saltum and at ever-shifting places, however steadily the tangential pressures producing it may act. Hence crushing alone may be shown to develope amply sufficient impulse to produce the most violent Earthquakes, whether they be or be not at a given place or time connected with volcanic outburst or possible injection, or with tangential pressures, enough still, in some cases, to produce partial permanent elevation.

When subterraneous crushing takes place, and the circumstances of the site do not permit the access of water, there may be Earthquake, but can be no Volcano; where water is admitted, there may be both.

And thus we discern why there are comparatively few submarine Volcanoes, the floor of the ocean being, on the whole, water-tight-"puddled," as an engineer would say, by the huge deposit of incoherent mud, etc., that covers most of it, and probably having a thicker crust beneath it than beneath the land.

We see, moreover, that the geological doctrine of absolute uniformity cannot be true as to Vulcanicity, any more than it can for any other energy in play in our world. Its development was greatest at its earliest stages, when the great masses of the mountain chains were elevated. It is even now-though as compared to men's experience, and even to all historic time, apparently uniform and always the same-a decaying energy.

The regimen of our planet as part of the Cosmos, which seems to some absolute (and presented to Playfair no trace of a beginning nor indication of an end), is not absolute, and only seems to us to be so because we see so little of it, and of its long perspective in time. This the now established doctrine of the conservation of energy renders certain.

With this source for volcanic heat, too, in our possession, we can look from our own world to others, and predict within certain limits, which must widen as our knowledge of the facts of their substance and surface becomes greater, what have been and what are the developments of Vulcanicity which have taken place or are occurring in or upon them. Looking to our own satellite, we see for the first time a sufficient physical cause for the enormous display of volcanic energy there which the telescope divulges to us; one which is not to be explained alone by the commonly made statement of the small density of the moon, but by the fact that as the rate of her cooling from a given temperature, as compared with that of our earth (apart from questions of the chemical nature of the two bodies, or of their specific heats, etc.), has been inversely as their respective masses, and directly as their surfaces, so has the rate of cooling of the moon been vastly greater than that of the earth, and the energy due to contraction by cooling more intense and rapidly developed in our satellite than upon our globe.

We have thus traced, in meagre and broken outline only-because space admitted no more-the progress of Science to its existing state as respects Vulcanicity, in its two branches of Vulcanology and of Seismology, and pointed out their more intimate relations and points of connection, and been at length able to refer them, on the sure basis of physical laws, to one common cause, and that one derived from no hypothesis, but simply from the postulate of our world as a terr-aqueous globe cooling in space.

What I have here advanced with reference to volcanic energy, which appertains to my own researches, I do not conceal from myself, nor from the reader, has yet to await the reception generally and the award of the true men of science of the world.

That, like every new line of thought which has attempted or succeeded in supplanting the old, it will meet with opposition, I make no doubt.

My belief, however, is that in the end it will be found to have added a fragment to the edifice of true knowledge.

The interpretation which I have given of the nature and origin of volcanic activity points at once to the function in the Cosmos which it is its destiny to fulfil. It is the instrument provided for the purpose of continually preserving the earth's solid shell in a state to follow down after the descending nucleus. It does this by an apparatus or play of mechanism whereby the material of the solid shell, locally or along certain lines, is not only crushed, but the crushed material is blown out as dust, or expelled as liquid rock from between the walls of the shell, which are thus enabled to approach each other; and thus, by relief of the tangential thrusts, to permit the shell to descend, which it is obvious that crushing alone, unless it extended to the whole mass of the shell, could not accomplish.

It is a wonderful example of Nature's mechanism thus to see how simple are the means by which this end is accomplished. The same inevitable crush that dislocates the solid shell along certain lines, produces the heat necessary to expel to the surface the material crushed.

When attempted to be made the basis for philosophic discovery, "final causes" are no doubt barren, as Bacon has said; but when we have independently and by strict methods arrived at a result, we may justly appeal, as a test of its truth, to its showing itself as plainly fulfilling a needful end, and, by a distinctly discernible mechanism, preserving that harmony and conservation which are the obvious law of the universe.

As has been said, if I mistake not by Daubeny, John Phillips, by Herschell, and by myself, the function of the Earthquake and the Volcano is not destructive but, preservative. But we now see that: that the preservative scope of this function, as respects our earth, is far wider than what has been previously attributed to it. The Volcano does not merely throw up new fertile soil, and tend, in some small degree, to restore to the dry land the waste for ever going on by rain and sea; it fulfils a far weightier and more imperative task; it-by a mechanism the power of which is exactly balanced to the variable calls demanded of it, and which working almost imperceptibly, although in a manner however terrible its surface-action may at times appear to us little men[G]-prevents at longer intervals such sudden and unlooked-for paroxysms in the mass of our subsiding earth's shell as would be attended with wide-spread destruction to all that it inhabit.

To the popular mind, Volcanoes and Earthquakes are only isolated items of curiosity amongst "the wonders of the world:" few geologists even appear to realise how great and important are the relations of Vulcanicity to their science, viewed as a whole. Yet of Vulcanicity it is not too much to say, that in proportion as its nature and doctrines come to be known and understood as parts of the Cosmos, the nearer will it be seen to lie at the basis of all Physical Geology.

* * *

[A] For a fuller account of the literature and history of advancement of human knowledge as to Earthquakes, here merely glanced at, I must refer to my First Report on the Facts of Earthquakes, "Reports, British Association, 1850," and to the works of Daubeny, Lyell, Phillips and others, its complete history remaining yet to be written.

[B] Yet how indistinctly formed were Young's ideas, and indistinct in the same direction as those of Humboldt, becomes evident by a single sentence: "When the agitation produced by an Earthquake extends further than there is any reason to suspect a subterraneous communication, it is probably propagated through the earth nearly in the same manner as a noise is conveyed through the air."-Lectures, Nat. Phil., Vol. I.

[C] The Right Rev. Charles Graves, F.R.S., etc., then Fellow of Trinity College, Pres. R. I. Acad., and now Bishop of Limerick, on presentation of the Academy's Cunningham Medal.

[D] In this Report, though I have never before referred to it, and do so now with reluctance, I have always felt that the Author did me some injustice. The only reference made to my labours, published the preceding year only, is in the following words: "Many persons have regarded these phenomena (viz., Earthquakes) as due in a great measure to vibrations ... and the subject has lately been brought under our notice, in a Memoir by Mr. Mallet, 'On the Dynamics of Earthquakes,' in which he has treated it in a more determinate manner, and in more detail, than any preceding writer" (p. 74). If that Paper of mine be collated with this Report, it will be, I believe, found that, as respects the earthquake part, the latter tint parades, in a mathematical dress, some portions of the general theory of earthquake movements, previously published by me as above stated. So, also, in the chapter (p. 90) referring to Seismometry, and the important uses to Geology that might be (and since have been, to some extent) made of it, no mention is made of those instruments previously proposed by me, nor of my anticipation of their important uses. This is but too mortifyingly suggestive of the-

"Pereant qui mea ante mihi dixerunt."

Having left this unnoticed for so many years, and during which the Author has preceded me to that bourne where our errors to each other must be forgotten, I should certainly not have now trespassed on the good rule, De mortuis nil nisi bonum, had I not observed very recently one amongst other results probably attributable to it. In Professor Phillips's "Vesuvius," if any one will refer to the passage beginning "The mechanism of earthquake movement has been investigated by competent hands. The late eminent mathematician, Mr. Hopkins, explained these tremors in the solid earth by the general theory of vibratory motion," etc. (pages 257-259)-I think he must, in the absence of collateral information, conclude that, not I, but Mr. Hopkins, was the discoverer of the Theory of Earthquakes as explained by the general theory of vibratory motion.

Probably my friend, Professor Phillips, had not recently referred to those Memoirs and Reports of twenty-four years back, and I am thoroughly convinced that, if he has here perpetuated an injustice, he has done so unintentionally and unwittingly.

Still, the facts show how true it is that

"The ill men do lives after them,

The good they do is oft interred with their bones."

And I may venture to ask my friend, should his admirable book reach, as I doubt not it will, another edition, to modify the passage.

[E] Assuming the point of ejection of this block (the crater) to be 8,000 feet above where it landed, and allowing it as high a density as admissible, and the angle of projection the best for large horizontal range, it may be proved that this mass, to reach nine miles horizontally, would require an initial velocity of projection of from 1,500 to 1,600 feet per second, one as great as that of a smooth-bore cannon-shot at the muzzle, and perfectly inconceivable to be produced by a volcano.

[F] The Rev. O. Fisher, M.A., F.G.S., in a most interesting and valuable Paper, "On the Elevation of Mountain Chains by Lateral Pressure, its Cause, and the Amount of it, with a Speculation on the Origin of Volcanic Action," read, April, 1868, and published in the Transactions of the Cambridge Philosophical Society, Vol. XI., Part III., in 1869, has deduced the necessary crushing of the earth's crust by a different but closely analogous method. I had not seen this Paper until after my own was in the hands of the Royal Society. The author's volcanic views are wholly different from my own, and do not appear to me equally valid with his notions as to elevation.-R. M.

[G] "Magna ista quia parvi sumus"-Seneca, "Qu?s. Nat."

END.

* * *

TRANSLATION

OF

PROFESSOR PALMIERI'S

ACCOUNT OF

THE ERUPTION OF VESUVIUS

OF

The great and disastrous conflagration of Vesuvius, which took place on the 26th of April, 1872, was, in my opinion, the last phase of an eruption which commenced at the end of January, 1871, an account of which I was unwilling to write, because I was convinced that it would not really terminate without a more or less violent explosion, such as I had often predicted. I shall now state the reasons upon which my prediction was founded.

When the central crater begins to heave, with slight eruptions, one may always predict a series of slight convulsions of greater or less duration, which are preparatory to the grand explosion, after which the Volcano remains for the most part in repose. Thus, when I observed the cone fissuring in November, 1868, and copious lava streams issuing from it, and flowing over the beautiful and fertile plains of the Novelle, through the Fossa della Vetrana, instead of announcing the beginning of an eruption, I announced the termination of one which had been manifest for upwards of a year by the constant flow of lava from the summit of the cone.

From the month of November, 1868, until the end of December, 1870, the mountain remained quiet, except that the fumaroles at the head of the fissure showed a degree of activity by which chlorides and sulphides of copper, sulphide of potash and other products, were engendered.

But in the beginning of 1871 the seismograph was disturbed,[1] and the crater discharged, with a slight detonation, a few incandescent projectiles. Then I announced that a new eruption had commenced, which might be of long duration, but with phases that could not possibly be foreseen; and on the 13th January, on the northern edge of the upper plain of the Vesuvian cone, an aperture appeared, from which at first a little lava issued, and then a small cone arose and threw out incandescent projectiles, with much smoke of a reddish colour, whilst the central crater continued to detonate more loudly and frequently. The lava-flow continued to increase until the beginning of March, without extending much beyond the base of the cone, although it had great mobility. In March, this little cone appeared not only to subside, but even partly to give way, as almost happens with eccentric cones when their activity is at an end. Upon visiting it, I observed that four prismatic or pillar-like masses remained standing, three of which were formed of scori? which had fallen back again in a pasty condition, and had become soldered together, the fourth consisting of a pyramidal block of compact and lithoidal lava, which appeared to have been forced up by impetus from the ground beneath. A little smoke issued from the small crater, and a loud hissing from the interior was audible. By lying along the edge, I could see a cavity of cylindrical form about ten metres in depth, tapestried with stalactitic scori? covered with sublimations of various colours. The bottom of this crater was level, but in the centre a small cone of about two metres had formed, pointed in such a manner that it possessed but a very narrow opening at the apex, from which smoke issued with a hissing sound, and from which were spurted a few very small incandescent scori?. This little cone increased in size as well as activity until it filled the crater, and rose four or five metres above the brim.[A] New and more abundant lavas appeared near the base of this cone, and, pouring continually into the Atria del Cavallo, rushed into the Fossa della Vetrana in the direction of the Observatory and towards the Crocella, where they accumulated to such an extent as to cover the hill-side for a distance of about 300 metres; then turning below the Canteroni, they formed a hillock there without spreading much farther. These very leucitic lavas are capable of great extension, the pieces which are ejected forming for the most part very fine filiform masses, which may be collected on the mountain in great quantities, and specimens of which I presented to the Academy under the name of filiform lapilli. These threads were often of a clear yellowish colour, and, when observed under the microscope, were found to consist of very minute crystals of leucite embedded in a homogeneous paste. The crystals were still smaller as the diameter of the threads was less, and never formed knots or swellings even in the most hair-like threads. These observations led me to reject the opinion of those who hold that crystals of leucite are pre-existent in the lava. The viscous nature of these lavas prevented their being covered with fragmentary scori?, but caused the formation at first of a skin, which, thickening, became at last a more or less pliable shell, that, when more solidified, allowed the still fluid part to run as in a tube formed of this solid shell. For many months the lava descended thus from the cone and traversed the Atria del Cavallo, always covered, appearing below the Canteroni of a lively fluidity, until it could no longer be enveloped in its skin, which was stretched by the addition of new lava, and finally rent asunder to give room to the current until, owing to diminished liquidity, it was constrained to stop. When the lava, having traversed the covered channel it had made for itself from the top of the mountain to below the Canteroni, made its appearance still running, it frequently formed large bubbles on the surface, which mostly burst to give vent to smoke, and then disappeared.

In October, 1871, near the edge of the central crater, another small crater was formed by falling in, which, after a few days, gave vent to smoke and several jets of lava. The principal cone frequently opened in some point of the slope to give egress to small currents of lava, which quickly ceased. But towards the end of October the detonations increased, the smoke from the central crater issued more densely and mixed with ashes, and the seismograph and accompanying apparatus were disturbed: for all these reasons, I said in one of my bulletins, we have either reached a new phase or the end of the eruption, not knowing whether the new phase would be the last. On the 3rd and 4th November copious and splendid lava streams coursed down the principal cone on its western side, but were soon exhausted. The cone of 1871 appeared again at rest, and partly even fell in, but did not cease to emit smoke and to show fire in the interior.

In the beginning of January, 1872, the little cone again became active, the crater of the preceding October resumed strength, with frequent bellowings and projectiles, and soon after lavas of the same kind as before reappeared. The cone of 1871, formed again by the lava ejected, became so full that the lava poured from its summit in the most singular and enchanting manner. So far only an eccentric or ephemeral cone had risen close to the central crater, which, after exhaustion, regained vigour and discharged lava from the apex instead of the base, as usually happens.

In the month of February matters were somewhat moderated; but in March, with the full moon, the cone opened on the north-west side-the cleavage being manifest by a line of fumaroles-and a lava stream issued from the lowest part without any noise and with very little smoke, and poured down into the Atria del Cavallo as far as the precipices of Monte di Somma. This lava ceased flowing after a week, but the fumaroles pointed out the cleft of the cone; and between the small re-made cone, which had risen to the height of 35 metres, and the central crater, a new crater of small dimensions and interrupted activity opened.

On the 23rd April (another full moon) the Observatory instruments became agitated, the activity of the craters increased, and on the evening of the 24th splendid lavas descended the cone in various directions, attracting on the same night the visits of a great many strangers. All these lava streams were nearly exhausted on the morning of the 25th; only one remained, which issued from the base of the cone, not far from the spot whence that of the preceding month had issued. Numbers of visitors, attracted by the splendour of the lava streams of the preceding night, which they supposed still continued, soon arrived, but, finding them exhausted, were for the most part conducted by their guides to see the one still flowing. It was almost inaccessible, and to reach it one had to walk over the rough inequalities of the scori?. It took me two hours to get there from the Observatory, when I visited it that morning, and therefore I endeavoured to dissuade those who wished to visit it at night from the attempt, but set out myself from the Observatory at 7 p.m., leaving my only assistant there. The instruments were agitated. After midnight the Observatory was closed, and my assistant retired to rest. Late and unlucky visitors passed unobserved with an escort of inexperienced guides; at half-past 3 o'clock in the morning of the 26th they were in the Atria del Cavallo, when the Vesuvian cone became rent in a north-westerly direction, the fissure commencing at the little cone which disappeared, and extending to the Atria del Cavallo, whence a copious torrent of lava issued. Two large craters formed at the summit of the mountain, discharging numerous incandescent projectiles with white ashes, and glittering with particles of mica, which frequently recurred.

A cloud of smoke enveloped these unfortunates, who were under a hail of burning projectiles and close to the lava torrent. Some were buried beneath it[B] and disappeared for ever; two dead bodies were picked up, and eleven grievously injured, one of whom died close to the Observatory. He alone revealed his name, Antonio Giannone. I learned afterwards that he was a fine young fellow, and Assistant-Professor in one of the Universities.

Assistant-Professors Signor Franco, who is a priest, and Signor Francesco Cozzolino, a priest also, entrusted with the festive mass for the Observatory, hastened to assist the dying. On my own return thither, the sad spectacle of the dead and dying awaited me; the former were conveyed, through the assistance of the municipal officer of Resina, to the Cemetery, and the latter to the Hospital. But we must leave this scene of grief and sorrow, and return to the eruption.

The fissure of the cone on the north-west side was large and deep, and extended into the Atria del Cavallo, about 300 metres. No mouth opened along the cleft of the cone itself; all the lava issued from that part which extended into the Atria. From previous experience I should have expected to have seen the formation of adventitious cones along the widest part of the fissure, which is never that most elevated, and these discharging from their summits ?riform matter frequently mixed with projectiles, and from their base lava; but on this occasion no cone appeared at the widest part of the fissure, but a long hillock was formed like a little chain of mountains, one point of which was elevated about fifty metres above the plain beneath, and bearing no resemblance to a cone.

Another fissure opened in the cone on the south side, which did not extend to the base, and lava issued from this and flowed in the direction of the Camaldoli. Streams of less importance furrowed the cone in other directions, but the largest quantity of lava proceeded from the fissure in the Atria del Cavallo, below the hillock or miniature chain of collines just described. This lava stream was for some time restrained within the Atria del Cavallo, among the holes and inequalities of the lavas of 1871, but these being filled up and overcome, it divided into two branches-the smaller one flowing through a hollow which separated the lavas of 1867 from those of 1871, and made its way over the lavas of 1858, threatening Resina, but stopped as soon as it reached the first cultivated ground; the larger branch precipitated itself into the Fossa della Vetrana, occupying the whole width, about 800 metres; and traversing the entire length of 1,300 metres in three hours. It dashed into the Fossa di Faraone; here it again divided into two streams, one overlying the lava of 1868, on the Plain of the Novelle, partially covering the cultivated ground and country-houses; the other flowing on through the Fossa di Faraglione, over the lava of 1855, reached the villages of Massa and St. Sebastiano, covering a portion of the houses, and thence continued its course through the bed of a foss or trench which, contrary to my advice, had been excavated after the eruption of 1855, in the expectation of diverting the course of that lava. I did not fail to observe that the rains which previously descended through these steep channels, would in future be kept back to filtrate through the scori?, without ever reaching the new channel.

The lava of this eruption, meeting with this said excavation, flowed into it, instead of pursuing its road over the lava of 1855, and thus invaded highly cultivated ground and towns of considerable value, extending to the very walls of a country-house belonging to the celebrated painter, Luca Giordano. This lava stream, having surmounted the obstacles which the heaps of scori? in the Atria del Cavallo presented to it, ran with great velocity (notwithstanding its being greatly widened out in the Fossa del Vetrano), so that between 10 a.m. and 11 p.m. it traversed about five kilometres of road, occupying a surface of five to six square kilometres. If it had not greatly slackened after midnight, from the failure of supply at its source, in twenty-four hours more, by occupying Ponticelli, it would have reached Naples, and flowed into the sea.

Although I had often visited the two villages of Massa and St. Sebastiano, previously greatly injured by the lava of 1855, yet I could not well estimate, upon now seeing them again, the number of houses which had disappeared. Massa seemed to me diminished by about one-third, and St. Sebastiano by somewhat less than a fourth. But the way of escape was open to the inhabitants of Massa; whilst a great river of lava occupying the road leading to St. Giorgio a Cremano would have hindered the flight of the inhabitants of St. Sebastiano, if they had been dilatory. The lava stream now separating the two villages is little less than a kilometre in width, and is about six metres in height.

On the night of the 26th April, the Observatory lay between two torrents of fire, which emitted an insufferable heat. The glass in the window-frames, especially on the Vetrana side, was hot and cracking, and a smell of scorching was perceptible in the rooms. The cone, besides being furrowed by the lava streams just described, was traversed by several others, which appeared and disappeared. It seemed completely perforated, and the lava oozed as it were through its whole surface. I cannot better express this phenomenon, than by saying that Vesuvius sweated fire. In the day-time, the cone appeared momentarily covered with white steam jets (fumaroles), which looked like flakes of cotton against the dark mountain-side, appearing and disappearing at brief intervals.

Simultaneously with the grand fissure of the cone, two large craters opened at the summit, discharging with a dreadful noise, audible at a great distance, an immense cloud of smoke and ashes with bombs and flakes, rising to the height of 1300 metres[C] above the brim of lava (sull' orlo de essi). The white ashes, before described, although they did not fall beyond the Crocella, were carried by the wind as far as Cosenza, from whence they were sent to me by Dr. Conti. These ejections were followed by dark sand, with lapilli and small fragments of scori? of the same colour. The smoke, driven up with violence, assumed the usual aspect of a pine tree, of so sad a colour that it reminded us of the shadowy elm of Virgil's dreams ("ulmus opaca ingens"). From the trunk and branches of the pine-tree cloud fell a rain of incandescent material, which frequently covered all the cone. The lapilli and the ashes were carried to greater distances.

The victims of the morning of the 26th, the torrents of fire which threatened Resina, Bosco and Torre Annunziata, and which devastated the fertile country of the Novelle, of Massa, St. Sebastiano and Cercola, the two partially buried villages, the continual and threatening growlings of the craters, caused such terror that numbers fled from their dwellings near the mountain into Naples, and several in Naples went to Rome or to other places. Very many delayed from the knowledge that I was in the Observatory, and held themselves in readiness for flight whenever I should abandon it.

The rapidity with which the vast torrent of fire assailed the houses (i.e., in these villages), and the great heat which spread to a distance, scarcely allowed the fugitives to carry away any of their belongings; many were completely destitute. The authorities vied with each other in zealous efforts to relieve the distress, and the municipality of Naples sheltered and fed the wretched beings for many days.

The igneous period of the eruption was short, for on the morning of the 27th the lava stream, bearing down upon Resina, having covered a few cultivated fields, stopped; the lava descending from the summit of the mountain towards the Camaldoli also stopped; and the great lava torrent, which passed the shoulders of the Observatory through the Fossa della Vetrana, lowered the level of its surface below those of its two sides, which appeared like two parallel ramparts above it.

If these streams had continued on the 27th, flowing in the same manner as they did on the night of the 26th, they would have reached the sea, bringing destruction to the very walls of Naples.

But before leaving the subject of these lavas I must narrate an important fact to which I was witness, and which was thrice repeated, near the banks of the great river of fire that ran close to the Observatory. At three several points, and at different times, I observed great balls of black smoke issue from the lava, driven up with continued violence, as if from a crater; through the smoke I frequently observed numerous projectiles thrown up into the air, but I could not say whether with noise or in silence, for the noise of the central crater was deafening. Each of these little eruptions, which I may call external eruptions, lasted from fifteen to twenty minutes. The first took place at the most elevated point of the Fossa della Vetrana, on the right bank of the torrent; the second, under the hill of Apicella, where the lava divided into the two branches, before described; and the third near to the Observatory on the left bank of the lava stream. These singular explosions terminated without leaving little cones or craters, the lava in its impetuosity carrying every trace away. These eruptions were seen from Naples, and the Observatory was justly believed to be in danger. One has been clearly photographed, the one which was the best seen from Naples, being the nearest and the least darkened by the smoke of the lava. (Plate 4.) Is this the first time that the phenomenon has been remarked? I believe that it is at least the first time it has been authenticated. The authority of Julius Schmidt, quoted by Scrope, has no weight with me, for I was also a witness of what happened at Vesuvius in 1855; and, although these cones were in the midst of the lava in the Atria del Cavallo, they originated, according to the opinion of everyone, from the fissure from which the other and much larger cones proceeded. The same phenomenon was observed in the Atria del Cavallo in 1858, when I caused two of the little cones to be brought to the Observatory; but these also might belong to the fissure along which the other cones were arranged. The same may be said of the little craters observed, after they had been exhausted, by Professor Scacchi in 1850. But the discharging mouths now observed in the Fossa della Vetrana, which existed for twenty minutes and then disappeared, and which were not at all in a continuous line, and could not be supposed to correspond with any fissure beneath, constitute a circumstance which, if not new, is evident for the first time, and cause the recognition of a power in the lava itself to form eruptive fumaroles.[2]

The igneous period of the eruption having terminated on the evening of the 27th, the ashes, lapilli, and projectiles became a little more abundant, whilst the roaring noises of the craters apparently became greater. The pine-tree cloud was of a darker colour, and was furrowed by continual lightning, visible by daylight from the Observatory. Many writers on the subject of Vesuvius affirm that the flashes which appear through the smoke cloud were lightning unaccompanied with thunder, but they studied the phenomena from Naples, or some place more or less distant from the crater, where the report of the thunder was inaudible, or could not be distinguished from the bellowing and detonation of the mountain. The fact is that these flashes were constantly followed by thunder, after an interval of about seven seconds.[D] When the flash was very short, a simple noise like the report of a gun was heard, but if it were long, a protracted sound like that from torn paper ensued.

On the 28th the ashes and lapilli, continuing to fall abundantly, darkened the air, yet without diminishing the terrible noise; at Resina, Portici, St. Giorgio a Cremano, Naples, etc., terror was universal.

On the 29th, with a strong wind blowing from the east, scori? of such a size fell at the Observatory, that the glass of the windows unprotected by external blinds was broken. The noise from the crater continued, but the projectiles rose to a less height, indicating a diminution in the dynamic power of the eruption. Towards midnight the noise of the craters was no longer continuous, and recurred with less force and for shorter intervals. Almost at the same hour a tempest burst over the Campania with loud thunder and a little rain. The grass, the seeds, the vine tendrils, the leaves and tops of the trees dried up immediately, and the country was changed from spring to winter. The storm, although repeated on the following days, passed away by degrees, and thus the floods, which I strongly feared, did not occur. Almost always after great eruptions of Vesuvius, storms of heavy rain have followed, and the ground being covered with ashes, the water could not filtrate through into the soil, but descended in muddy torrents over the adjacent country, occasioning as much damage as the fire itself.

On the 30th, the detonations were very few, and the smoke issued only at intervals, and by the 1st May the eruption was completely over.

When the smoke had cleared off the figure of the cone was seen to be changed. (Vide Plate 5a.)

The ground was perpetually disturbed whilst the Volcano raged, so that the Observatory oscillated continually. Some shocks were felt not only in the adjacent territory, but at a greater distance, at Montovi and elsewhere. The oscillations at the Observatory were chiefly undulatory, from N.E. to S.W. They were observed for some days after the termination of the eruption, but not continuously, although they maintained some intensity.

If we refer to January, 1871, we shall find that that eruption was preceded by several earthquakes, among which were those of the months of October, November and December, in the previous year, that wrought such destruction in Calabria, and especially in the province of Cosenza; if we consider that as only the last phase, we shall find that it was preceded by great shocks of earthquake that devastated some regions of Greece.[3]

The great quantity of lapilli which fell buried the scori? with which the Vesuvius cone was covered, so that it became somewhat more difficult to ascend to the summit, and much less difficult to descend. Having reached the top of the mountain, I found a large crater divided into two parts by what seemed a cyclopean wall. The two abysses had vertical sides, and revealed the internal structure of the cone. Their vertical depth was 250 metres; and beyond that I observed a sort of tunnel perforated in the rock, with a covering arch raised above the bottom of the eastern abyss about 12 metres, judging by the eye. The interior walls of the crater showed neither the usual stalactitic scori? nor sublimations, nor fumaroles, but alternate beds of scori? and of compact lava. The fumaroles and sublimations abounded, only about the brims of the craters. Hydrochloric and sulphuric acid and sometimes sulphuretted hydrogen affected respiration, and the temperature rose sometimes to 150 degrees. Various fissures about the brim of the double crater indicated prolongations downwards, which allowed me to descend with a rope, in order to examine the interior of the tunnel to which I have just alluded. The highest brim of the crater was fissured for a distance of 80 metres, and the greatest depth of fissure was at that place.

By measurement with the barometer, we ascertained approximately (for only one barometer was used) that the height of the Vesuvian cone was somewhat diminished.

Not only the Vesuvian cone, but the whole adjacent country appeared white for many days, as if covered with snow, when exposed to sunlight. This was due to the sea-salt contained in the ashes with which the surface was strewn.

A great quantity of coleoptera assembled on the flat roof of the Observatory, where the ashes and lapilli were heaped up two decimetres in height. I found the same species on the cone, where many insects were observed on other occasions, such as the Cuccinella septempunctata; the crysomela populi, etc., were wanting. This phenomenon of the extraordinary concourse of insects on the top of Vesuvius, in order to die in some of the fumaroles, especially noted previous to and after great eruptions, is a circumstance for which I cannot account.[4] The whole of the lava emitted in this eruption occupies a surface of about five square kilometres; allowing an average thickness of four metres, we obtain a mass of twenty millions of cubic metres. About three-fifths of this lava did no injury, being deposited upon other pre-existing lava. However, the lava in the Novelle, which was deposited upon the lava of 1858, covered quarries of the best stone which had been worked at the time, covered many paths that had been cleared, and buried the new Church of St. Michele, with some houses that surrounded it, which had been rebuilt on the site of the former church, which was covered by the lava of 1868. The destruction of land in occupation, of buildings and of crops, exceeded three million francs in value. Many proposals for relieving the sufferers have been received. Wishing to aid in this benevolent work, I gave a public lecture, admission for each person being one franc; and this lecture, from notes badly taken, was printed by private speculation, and I was compelled to repudiate the report of it through the public papers.

The evolutions of carbonic acid (mofette), which usually appear at the end of great Vesuvian eruptions at low-situated spots or hollows, with very rare exceptions, were observed on this occasion a few days after the eruption had completely ceased. They appeared in the direction of Resina. I found the most elevated at Tironi, and the most numerous between La Favorita and the Bosco Reale di Portici.

The water in wells was on this occasion neither deficient nor scarce previous to the eruption, but was very acid after the appearance of the carbonic acid evolutions in those neighbourhoods in which they abounded. Having stated that the disastrous conflagration of the 26th April ought, in my opinion, to be regarded as the last phase of a long period of eruption, which commenced at the beginning of 1871, I consider it right to discuss the question at somewhat greater length.

Not only from twenty years' personal observations, but from the attentive study of accounts of previous eruptions, I have found that when the central crater awakens with small eruptions after a certain time of previous repose, these almost always have a long duration, and, after various phases of increase and decrease, terminate in a great eccentric eruption, that is to say, with the production of an aperture from which a copious lava stream issues. The eruptions of 1858, 1861, 1868 and 1872, furnish the most recent examples of what I affirm. I might cite many others of earlier date, but I shall content myself with recording the greatest conflagration of this century, that of October, 1822.

Before the erection of the Vesuvian Observatory, it was impossible to obtain a consecutive account of all the phases which the Volcano presented; but we generally obtained the description of the more splendid phases of the eruption which arrested the attention of everyone. Hence, notices of the small phenomena which preceded a great eruption are frequently wanting. We cannot always ascertain whether the fumaroles of the craters became active and at what periods, what was their temperature and what the diverse nature of their emanations, etc.: whether and when any change in the crater with slight eruptive manifestations occurred; discharges which sometimes commenced in the bottom of a crater becoming active, and so are invisible at Naples.

But it may be asked whether the inverse proposition be equally true, that is, whether all the great eruptions of our Volcano were preceded by small fiery manifestations of long duration? There have undoubtedly been great eruptions not preceded by small central eruptions, but these also had their period of preparation or precursory signs. After the great eruption of 1850, Vesuvius remained in apparent repose until the end of May, 1855, when there was an eccentric eruption and a great flow of lava lasting twenty-seven days. But for a year before the fumaroles on the top of the mountain had acquired great activity, their temperature increased, and hydrochloric and sulphuric acid became more abundant, and generated the usual coloured products on the adjacent scori?. Finally, in the month of January, a crater was formed by falling in of the ground, and although it did not discharge fire, yet it poured forth dense smoke. This was the beginning of the fissure manifested four months afterwards.

Ignazio Sorrentino, who spent a long life in the study of Vesuvius, and frequently ascended it, considered the increase of those yellow products-which are chiefly chlorides of iron, but were, at that time, mistaken for sulphur-as the sign of an approaching eruption.

The only grave objection that can be alleged is that of the memorable eruption of 1631, which surprised the neighbouring population so suddenly that many perished miserably, surrounded or covered with lava. But that terrible conflagration occurred after centuries of repose, so that trees had grown in the interior of the crater. No one suspected the possibility of danger. It took place, too, at the end of autumn, when the cone is usually covered with clouds, and, therefore, no one had an opportunity of observing any precursory phenomena.

When the Observatory was established, I was able-in the first instance, at my own expense, and afterwards with some slight assistance from Government-to undertake studies more assiduous than any previously made. I had two instruments adjusted to indicate the internal efforts of the Volcano, viz., M. Lamond's apparatus of variations, which, by means of finely-balanced needles and methods of amplification proposed by Gauss, indicates the slightest trepidation of the ground, and my own electro-magnetic seismograph, a self-registering instrument of exquisite delicacy. These instruments, when attentively observed, give the most valuable information with respect to the activity of the adjacent Volcano.

If the very slightest eruption occurs, these instruments manifest slight perturbation, increasing with the activity of the mountain. When the Volcano attains a certain degree of activity, and the instruments are proportionately disturbed, it is impossible to foresee a new phase of increase without constantly watching the changes in the intensity of the perturbations; and to effect this it is requisite to have upon the spot a staff of assistants sufficiently numerous, scientific and intelligent. If, therefore, on the night preceding the 26th of April the instruments had been properly watched, they would have undoubtedly indicated the great increase in the activity of the Volcano. The perturbations on the 23rd were steadily increasing, and on the evening of the 25th they were much stronger than on the 24th, but on the morning of the 26th they had become extraordinarily strong; they must, therefore, have increased considerably during the night.

* * *

When the observer is near the source of the lava, he sees matter in a state of fusion, which, like a torrent of liquid fire, runs along, with more or less impetuosity, between two banks formed by itself. But as soon as the surface of the torrent cools to the point of congelation, it loses the splendour of its first incandescence.

The part which begins to harden breaks readily in some lavas into fragments which float on the viscous fluid beneath; these, increasing in number with distance from the source, conceal the molten matter beneath and retard its progress, and at last nothing is seen but the more or less red-hot scori? moving along. These lavas I shall call "Lavas with fragmentary scori?."

On other occasions, a skin forms on the surface of the lava, which, gradually thickening, keeps flexible for some time, and then wrinkles or swells or extends and breaks to give egress to the hot fluid within, which, in its turn, skins over and repeats the same phenomena. This I shall call "Lavas with a united surface."

These, in their course, discharge less smoke than the first, draw out more easily into threads, and, when cold, have a dark colour, something like bitumen or pitch. The lava with fragmentary scori?, when stretched, breaks easily, discharges smoke copiously, and, when hardened, has a more bluish tint, like clods of upturned earth (formato di zolle). It is noisy in its course, because the incoherent scori? that it carries along strike and crunch against each other; the other lava flows silently, except for a sort of crackling arising from the actual fracturing up of the solid skin by distension from the liquid matter within. If required to give the mineralogical characteristics of this lava, I would say that it was rich in leucite and contained little or no pyroxene; the fragmentary lava, on the contrary, is poor in leucite and rich in pyroxene. The lavas of 1871 were of the "united surface" character; those of 1872 were "fragmentary," with some characteristics which I shall describe:

1. They were of the clearest tint I have ever seen, when regarded superficially, but, when broken, the fracture was darker than any other lava.

2. They had very little leucite and abounded in pyroxene and olivine, and sometimes contained a few crystals of amphibole.

3. Their specific gravity varied with their porosity; the most compact attained 2·75.

4. These lavas carried along in their course a quantity of scori? which had long been subjected to the action of the acids of the fumaroles close to the craters, and also a great many bombs (bombe)-that is, round masses similar to those ejected from craters. These varied in size, some having a diameter of four to five meters. They frequently contained a large nucleus of very leucitic lava, like that of 1871, with a larger or smaller quantity of feroligiste (peroxide of iron). Others contained lavas changed by the action of the acid vapours near the craters. These bombs must have flowed out with the lava, for they are found through its whole course, and they were certainly not ejected from the crater; for not only are they found on the lava exclusively, but masses so enormous were not thrown up from the craters during the eruption; those lying on the cone near the craters seldom exceed a decimetre in diameter.

As to the qualitative chemical analysis of the lavas, it always presents the same elements, with the exception of small quantities of some metals, lead for example, which have escaped the researches of good chemists, but which I have constantly found in the sublimations of the fumaroles of the lava. With respect to the quantitative analysis, two specimens of the same lava appear indeed to have their constituents in different proportions. To arrive at any conclusion a long and patient investigation, requiring means and assistance which the Observatory does not possess, would be necessary.

Professor Fuchs, of Heidelberg, has devoted himself to this work for years past, and if he continue it with well-selected and sufficiently large specimens we may hope some day to obtain satisfactory results.

5. Every specimen of lava which I examined with a very sensitive magnetoscope improved by myself, was invariably magneto-polar, not excepting the pieces of the bombs, whether rejected from the crater or carried along with the lava.

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