The S. P. 888 was shaking throughout her structure before she came square with the exit of the cove. If a destroyer is "a tin box built around a mighty big engine," the term even more nearly fits one of these chasers.

The four Navy boys from Seacove were amazed by the quickness with which she got under way and the brief time it took to tune her up to top-notch speed.

"She's a hundred and ten feet long," said Mr. MacMasters, "about as wide as a happy thought, and can make her thirty-five knots an hour without any particular effort."

"No effort?" muttered Torry. "And it feels as though she was shaking herself to pieces!"

"She's faster than the Colodia," observed Whistler, somewhat as though he felt pained by that fact. That any other craft should be a sweeter sailer than his beloved destroyer seemed to him almost a crime.

"She most certainly is," agreed Ensign MacMasters. "She is some speed boat!"

"Why!" Frenchy cried, "she must be faster than the admiral's hydroboat we saw at Newport."

"No, no!" said the ensign. "Those hydroboats have got every other craft in the Navy beaten to a standstill. And about all they use 'em for is pleasure boats."

"They'll be dispatch carriers maybe?" suggested Whistler.

"What do they want of dispatch carriers in a day of wireless?" returned the ensign, and went about his duty of conning the S. P. 888 as she shot through the breach between the claw-like capes that defended the cove, and so straight out to sea in a southeasterly direction.

The "bone in her teeth," as sailors call the white water under the ship's bows, became a windrow of sea, foamed-streaked and agitated, parted by the knife-sharp bows, and rolling away on either hand. The S. P. 888 traveled so swiftly that at a distance "shark" really was the name for her.

She was not camouflaged, as were the hull and upperworks of many Navy vessels with which the four friends were familiar; but her dull coloring made her well nigh unobservable at a few miles' distance when she lay at rest. When she was in action no amount of deceiving paint would hide her, because of the water she disturbed.

The motor boat Phil had suspected had more than an hour and half's start. If she had kept straight ahead on the course she was going when last observed by the boys, she must now be twenty miles or more off shore.

The chaser, propelled by her powerful engines, could traverse that distance, and the oil boat's additional miles, in less than two hours. If the pursued vessel did not change her course she could be easily overtaken before twilight.

Ensign MacMasters was too busy to talk further with the four chums; indeed it would not be conducive to discipline for the commissioned officer to give the apprentice seamen too much of his attention.

But Mr. MacMasters and the four Seacove boys had been through some warm incidents together; and there is always a particular bond between those who have been shoulder to shoulder in a good fight.

"Remember the rumpus we had, Mr. MacMasters and us fellows, when those Germans tried to recapture the Graf von Posen?" Ikey asked his mates.

"Are we likely to forget it?" retorted Al.

"What about it, Ikey?" asked Michael Donahue, complacently. "It was a lovely fight!"

"Do you s'pose the fellows on this oil tender we are chasin' will fight?" asked Ikey.

"Not a chance. Here's fifty men on this chaser. The Germans-if they are Germans-wouldn't stand any show. There are only a few of them," said Torry.

"Including the black-whiskered chap Whistler tells about," Frenchy said. "Hey, Whistler!"

"What is it?" asked the older lad seriously.

"D'you really think that power boat we saw is going out to meet a submarine?"

"Ask me an easier one," said Morgan. "I can't guess. But she might. We know very well that German submarines and German raiders, and even Germany itself, pass news back and forth by wireless. We can't control the vibrations of the air-worse luck!"

"Now you've said something, boy!" agreed Torry.

"They read all the news that passes between our ships, too, unless it is in a secret code. And they pick everything they need to know about our ship movements out of the air."

"Too bad wireless was ever invented, then," grumbled Torry.

"Six of one and half a dozen of the other," grinned Frenchy. "You bet our operators steal German messages."

"It's likely. You know that chap on the Colodia whom we all liked so well, the chief wireless operator, got lots of information that was supposed only to be picked up by German submarines.

"In this case," added Whistler Morgan, "the sub may have wirelessed word for supplies. We don't know how many alien enemies may be running wireless stations in the United States. The Secret Service men are unearthing them all the time."

"Well," sighed Ikey, "I only hope we'll catch up with this oil tub we're hunting just as she is unloading her cargo onto a sub. Then! Blooey! We'll drop a depth bomb or two, and settle Mr. Submarine."

"Just like that!" drawled Whistler. "It sounds easy. How many times did the Colodia chase a U-boat and lose it?"

"Crickey!" breathed Torry, "even the Colodia couldn't travel like this shark."

"Oh! you admit it, do you?" grinned Frenchy. "Well, we are going some!"

But there was an element working against the S. P. 888-an element which could not be controlled. No matter how speedy the oil boat might have been, the chaser could have overtaken her had she kept a straight course. That was understood.

But the farther they went the more certain it was that this new element was going to balk them. It was fog. The horizon was masked by it, and soon the damp feel of it was upon them.

Mr. MacMasters paced the deck anxiously. Not a smudge of smoke did he or the lookouts raise. But the growing fog cloud would soon have hidden anything of the kind, even if the oil boat had been near at hand.

"Fog-haunted, Morgan," he said to Whistler, with disappointment. "We'll run on for a while; but it is hopeless, I guess. You say you know one of the men aboard that power boat?"

Morgan told him what he knew of the bewhiskered man called Blake; and also of the little water wheel that was whirling under the waterfall at the Elmvale Dam, although really, it did not seem to him as though that little invention could have a serious connection with any alien-enemy activities.

"I will report the whole thing," Mr. MacMasters said. "But, of course, the Department receives similar and even less assured testimony every day, of suspiciously acting persons. The information furnished the Department has all to be sifted. There may be nothing wrong with this man Blake."

"If he is working at the munition factory, how comes it that he is out here on an oil-laden boat?" demanded Whistler, with what he thought was shrewdness.

"Quite so. You boys are naval apprentices, but you were out fishing to-day," returned Mr. MacMasters, grimly. "There is an explanation for everything, my boy."

They ran on for another hour, but more slowly. They did not raise a craft of any kind, and Mr. MacMasters lost hope.

"I will put you boys ashore at Rivermouth," he said. "You can go home by rail. I shall not be able to put in at Seacove again to-night. And Rivermouth is off yonder-within a few miles."

Even in the fog the navigator found the harbor in question without difficulty. Just as they would have apprehended the presence of a submarine had one been near. There are very delicate and wonderful instruments aboard American naval vessels-instruments that may not be described at present-that enable the officers to apprehend the near approach of other vessels and their own nearness to the shore as well.

The S. P. 888 made her landfall correctly and slipped into Rivermouth Harbor like a ghost in the fog. There was a quantity of small shipping in the place, and Ensign MacMasters did not want to take any chances of collision. So he hailed a fishing smack and put the four friends from Seacove aboard of her.

"Good-bye, boys!" he said, as they went over the side into the smack. "We shall meet in a few days. You will get your notice by telegraph when to join the Kennebunk, and where. I shall be relieved from the command of this shark, and we'll have a big cruise on the superdreadnaught, I have no doubt."

He spoke prophetically, as it was proved later. But at this time neither Ensign MacMasters nor any of the four apprentice seamen imagined just how wonderful a cruise it would be.

As the fishing smack chugged away with her auxiliary engine toward the docks of the town, the S. P. 888 swung in a narrow circle and put out to sea so swiftly that in five minutes she was completely out of sight in the fog and almost out of sound as well.

The fishermen were curious about the boys and the business of the chaser in this locality; but the Navy boys had long since learned to say nothing that would circulate information of any moment. "Keep your mouth closed" is an inflexible rule of the Navy; the yarns Ikey told his "papa" and his "mama" notwithstanding!

As they drifted in toward shore slowly, weaving their way among the moored craft, Whistler suddenly began to sniff the air and show excitement.

"What's the matter?" demanded Torry, his closest chum. "You act like a hound dog on a hot scent."

"Or a colored gem'man smelling po'k chops on the frypan," suggested Frenchy, chuckling.

"Say, Mister," asked Whistler, turning to the skipper of the smack, "is there a tank ship in here?"

"An oil tanker? No! Nothing like it."

"I smell it, too!" exclaimed Ikey suddenly.

"What you boys smell is the Sarah Coville that came in just ahead of us. She's anchored here somewhere," said the fisherman.

"What sort is she?" Whistler demanded. Then he described swiftly the oil tender he had marked that afternoon passing the Blue Reef fishing grounds.

"That's her," said the man. "She often slips in here. Don't know who owns her now. Used to belong to the Texarcana Oil Company before the war. She's only a lighter."

"Is she laden?" asked Whistler.

"Didn't look so to me," was the reply.

Whistler Morgan said no more, and he warned his friends to have no further talk upon the matter. After they got ashore, however, all four were much excited by the incident.

"She was loaded to the Plimsoll mark when she passed us," Torry said. "What could she have done with her cargo in so short a time?"

"I'd like to know," agreed Whistler thoughtfully.

"We ought to tell somebody," declared Frenchy.

"Let's be sure we tell the right person," Whistler advised. "Come on now and get some supper. We've an hour to wait for a train to Seacove."

They marched up the main street of the port. The fog was not so thick inshore here. Just before they reached the restaurant they usually patronized when they were in the town, Whistler uttered an exclamation and held his friends back.

"See those two men going into Yancey's Restaurant?" he queried.

"What about 'em?" Frenchy asked.

"The fellow ahead," said Whistler Morgan deeply in earnest, "is that man Blake. The other I bet is the captain of the Sarah Coville."

"Well," asked Torry, after a moment, "what are you waiting for? Their eating at Yancey's won't stop us from going there too, will it?"

* * *