Chapter 7 -A PUZZLING PROBLEM.

It is now time that we returned to the island where we left Pudge Perkins patrolling the beach, and Frank Chester and Billy Barnes wrapped in slumber. Frank had set the alarm clock for midnight, when it had been arranged that he and Billy were to turn out on patrol, and its insistent clamor had only just commenced when he sprang out of his bunk broad awake and prepared to go on duty. Billy stretched and yawned a bit before he, too, tumbled out.

"Gee whillakers!" he exclaimed, as he got into his clothes, "it seems to me that we are making a lot of fuss over nothing, Frank. I don't believe those fellows will come near the island to-night."

"Perhaps not; but it's our duty to be on guard. If anything happened to Dr. Perkins' invention now it would be almost impossible to repair it in time for the tests he wants to make."

Talking thus the two lads got into their clothes, drank some coffee, which Frank had prepared while they were dressing, and then set out into the night. They made for the cove from which Harry had started his eventful swim.

"Best wait here till they come round," said Frank, and he and Billy found places in the sand and made themselves as comfortable as possible till they should hear the footsteps of one of the young sentries. They had not long to wait. Hardly fifteen minutes had elapsed before Frank's sharp ears caught the sound of some one approaching. A minute later Pudge joined them. His first words were not calculated to make the newcomers feel at ease.

"Where's Harry?" he demanded.

"Don't you know?" ejaculated Frank with considerable surprise.

"No. I've been making my patrol regularly, and the last three times I've been round I haven't met him."

Frank's face could only be dimly seen in the darkness, but all his alarm was plain enough in his next words.

"What can have become of him?"

"Maybe he took the dinghy and decided to look over the motor boat and the hulk," suggested Billy.

"That's easy enough to find out," declared Frank, starting for the place where the dinghy had been beached. A moment later he stumbled over the anchor and, closely following this, by the aid of a lighted match, he made the discovery that the rope had been slashed.

"Harry never took that dinghy," he exclaimed apprehensively, "there's been some crooked work here."

"Thunder and turtles! What do you mean?" gasped Pudge, fully as anxiously.

"That some one has landed here and stolen the dinghy and taken Harry along with them. I can't think of any other explanation. Harry would never have cut that rope."

"You mean he's been carried off?" The question came from Billy Barnes.

"I can't think of any other explanation. Pudge, did you hear anything that sounded suspicious?"

"Oilskins and onions, no! Not a sound. Let's fire a pistol and see if we get any answer."

"That's a good idea, Pudge-Great Scott!"

"What's the matter?" demanded Billy Barnes, as Frank broke off short and uttered the above exclamation.

"Look here! Harry's clothes! Wait till I get a light. There! Now, see all his outer garments and his pistol lying by them."

"Gatling guns and grass hoppers, if this doesn't beat all."

"He can't have been carried off, then," burst out Billy, "but if he wasn't, how did that dinghy rope come to be cut?"

Frank made no answer at the moment. The discovery of Harry's clothes on the beach had put a dreadful fear into his mind. What if the boy had heard a disturbance on the hulk or on the motor boat and, having swum off to see what was the trouble, had been seized with a cramp and drowned?

But Frank firmly thrust the question from him the next minute. Such thoughts were by far too unnerving to be dwelt on. The others remained silent. They seemed to be waiting for Frank to speak. Presently the words came.

"It's too dark to see anything out there," said the boy, in as firm a voice as he could command. "Let's fire three shots-the signal we agreed upon-and then if Harry is on the hulk or the motor boat he will be sure to answer them."

The others agreed that this seemed about the best thing to do, and Pudge, taking Harry's discarded weapon, fired it three times. Then came a long pause, filled with an ominous silence.

"Try again," said Frank in a strained voice. Once more three sharp reports sounded. But again there was no answer.

"That settles it," declared Frank solemnly; "something has happened to Harry. We must get out to the hulk and to the motor boat."

"How? The dinghy's gone, and--"

"I'm going to swim for it."

Already Frank had thrown off his outer garments. On the beach lay a balk of timber which they sometimes used to tie the dinghy to. Frank now ordered his companions to help in rolling this down to the water.

"I'm going to use it as a help in swimming out there," he said; "the water's pretty cold, and I don't want to risk a cramp."

"Wait till daylight, Frank," urged Billy; "it won't be long till dawn now, and--"

But Frank cut him short abruptly.

"My brother's out there somewhere," he said in a sharp, decisive voice, "and I'm going to find out what's happened to him."

A minute later Frank was in the water pushing the balk of timber before him and heading, as nearly as he knew how, for the spot where the hulk and the motor boat had been moored.

It was more than half an hour before Billy and Pudge saw him again. Then he reappeared, chilled through and shivering in every limb. His first words almost deprived his companions of breath.

"They're gone!" he exclaimed.

"What!" the exclamation came from both Billy and Pudge simultaneously. They guessed by some sort of intuition what Frank referred to.

"Yes, they're both gone," repeated Frank; "the Betsy Jane and the motor boat."

"Are you sure you're not mistaken, Frank?" inquired Billy, unwilling to believe the extent of the catastrophe that had overtaken them.

"I'm as sure that they're gone as I am that I am standing here," was the reply. "I cruised about on my log for quite a radius, and couldn't discover a sign of them. I found the motor boat's buoy, though. She had been untied by some one."

"But the Betsy Jane? Schooners and succotash! The Betsy Jane!" broke in Pudge.

"Gone, too," Frank's voice broke, "but I wouldn't care about either if I only knew what had become of Harry."

"Come on up to the hut and we'll have some hot coffee and talk it over," said Billy, who saw that Frank, besides being almost numb with cold, was half crazy at the mystery of Harry's fate.

Frank suffered himself to be led up to the hut and the rest of the night was passed in speculation as to the fate of the missing boy. All three of the lads were pretty sure that the two Daniels had had a hand in the night's work somehow, but they were far from guessing what had actually occurred.

Soon after daylight the wireless began working. Dr. Perkins notified them from Portland that he expected to arrive that afternoon at Motthaven, and wished them to meet him. Frank found some relief for his wrought-up feelings in informing the inventor of what had occurred.

"Will charter fast boat and be there with all speed," came the reply through the air; "make the best of it till I come. Am confident that everything will come out all right."

And with this message the "marooned" trio on the island had to be content. The day was passed in making a careful survey of the island to discover, if possible, some trace of the marauders. But none was to be found. The tide had even obliterated any footmarks they might have left in the damp sand. Thoroughly disheartened and miserable, the boys ate a scanty lunch and then sat down to await the arrival of Dr. Perkins.

It was sundown when a fast motor boat appeared to the southward, cleaving the water at a rapid rate. A quarter of an hour later Dr. Perkins was hearing from the boys' own lips the strange story of their adventures of the past day and night.