not for long did the young inventor endeavor to break his way out of the water-ballast tank by striking the heavy sides of it. tom realized that this was worse than useless. he listened intently, but could hear nothing. even the retreating footsteps of andy foger were inaudible.

"this certainly is a pickle!" exclaimed tom aloud. "i can't understand how he ever got here. he must have traced us after we went to shopton in the airship the last time. then he sneaked in here. probably he saw me enter, but how could he know enough to work the worm gear and close the door? andy has had some experience with machinery, though, and one of the vaults in the bank where his father is a director closed just like this tank. that's very likely how he learned about it. but i've got to do something else besides thinking of that sneak, andy. i've got to get out of here. let's see if i can work the gear from inside."

before he started, almost, tom knew that it would be impossible. the tank was made to close from the interior of the submarine, and the heavy door, built to withstand the pressure of tons of water, could not be forced except by the proper means.

"no use trying that," concluded the lad, after a tiring attempt to force back the sliding door with his hands. "i've got to call for help."

he shouted until the vibrations in the confined space made his ears ring, and the mere exertion of raising his voice to the highest pitch made his heart beat quickly. yet there came no response. he hardly expected that there would be any, for with his father and mr. sharp away, the engineer absent on an errand, and mrs. baggert in the house some distance off, there was no one to hear his calls for help, even if they had been capable of penetrating farther than the extent of the shed, where the under-water craft had been constructed.

"i've got to wait until some of them come out here," thought tom. "they'll be sure to release me and make a search. then it will be easy enough to call to them and tell them where i am, once they are inside the shed. but—" he paused, for a horrible fear came over him. "suppose they should come—too late?" the tank was airtight. there was enough air in it to last for some time, but, sooner or later, it would no longer support life. already, tom thought, it seemed oppressive, though probably that was his imagination.

"i must get out!" he repeated frantically. "i'll die in here soon."

again he tried to shove back the steel door. then he repeated his cries until he was weary. no one answered him. he fancied once he could hear footsteps in the shed, and thought, perhaps, it was andy, come back to gloat over him. then tom knew the red-haired coward would not dare venture back. we must do andy the justice to say that he never realized that he was endangering tom's life. the bully had no idea the tank was airtight when he closed it. he had seen tom enter and a sudden whim came to him to revenge himself.

but that did not help the young inventor any. there was no doubt about it now—the air was becoming close. tom had been imprisoned nearly two hours, and as he was a healthy, strong lad, he required plenty of oxygen. there was certainly less than there had been in the tank. his head began to buzz, and there was a ringing in his ears.

once more he fell upon his knees, and his fingers sought the small projections of the gear on the inside of the door. he could no more budge the mechanism than a child could open a burglar-proof vault.

"it's no use," he moaned, and he sprawled at full length on the floor of the tank, for there the air was purer. as he did so his fingers touched something. he started as they closed around the handle of a big monkey wrench. it was one he had brought into the place with him. imbued with new hope he struck a match and lighted his lantern, which he had allowed to go out as it burned up too much of the oxygen. by the gleam of it he looked to see if there were any bolts or nuts he could loosen with the wrench, in order to slide the door back. it needed but a glance to show him the futility of this.

"it's no go," he murmured, and he let the wrench fall to the floor. there was a ringing, clanging sound, and as it smote his ears tom sprang up with an exclamation.

"that's the thing!" he cried. "i wonder i didn't think of it before. i can signal for help by pounding on the sides of the tank with the wrench. the blows will carry a good deal farther than my voice would." every one knows how far the noise of a boiler shop, with hammers falling on steel plates, can be heard; much farther than can a human voice.

tom began a lusty tattoo on the metal sides of the tank. at first he merely rattled out blow after blow, and then, as another thought came to him, he adopted a certain plan. some time previous, when he and mr. sharp had planned their trip in the air, the two had adopted a code of signals. as it was difficult in a high wind to shout from one end of the airship to the other, the young inventor would sometimes pound on the pipe which ran from the pilot house of the red cloud to the engine-room. by a combination of numbers, simple messages could be conveyed. the code included a call for help. forty-seven was the number, but there had never been any occasion to use it.

tom remembered this now. at once he ceased his indiscriminate hammering, and began to beat out regularly—one, two, three, four—then a pause, and seven blows would be given. over and over again he rang out this number—forty seven—the call for help.

"if mr. sharp only comes back he will hear that, even in the house," thought poor tom "maybe garret or mrs. baggert will hear it, too, but they won't know what it means. they'll think i'm just working on the submarine."

it seemed several hours to tom that he pounded out that cry for aid, but, as he afterward learned, it was only a little over an hour. signal after signal he sent vibrating from the steel sides of the tank. when one arm tired he would use the other. he grew weary, his head was aching, and there was a ringing in his ears; a ringing that seemed as if ten thousand bells were jangling out their peals, and he could barely distinguish his own pounding.

signal after signal he sounded. it was becoming like a dream to him, when suddenly, as he paused for a rest, he heard his name called faintly, as if far away.

"tom! tom! where are you?"

it was the voice of mr. sharp. then followed the tones of the aged inventor.

"my poor boy! tom, are you still alive?"

"yes, dad! in the starboard tank!" the lad gasped out, and then he lost his senses. when he revived he was lying on a pile of bagging in the submarine shop, and his father and the aeronaut were bending over him.

"are you all right, tom?" asked mr. swift.

"yes—i—i guess so," was the hesitating answer. "yes," the lad added, as the fresh air cleared his head. "i'll be all right pretty soon. have you seen andy foger?"

"did he shut you in there?" demanded mr. swift.

tom nodded.

"i'll have him arrested!" declared mr. swift. "i'll go to town as soon as you're in good shape again and notify the police."

"no, don't," pleaded tom. "i'll take care of andy myself. i don't really believe he knew how serious it was. i'll settle with him later, though."

"well, it came mighty near being serious," remarked mr. sharp grimly. "your father and i came back a little sooner than we expected, and as soon as i got near the house i heard your signal. i knew what it was in a moment. there were mrs. baggert and garret talking away, and when i asked them why they didn't answer your call they said they thought you were merely tinkering with the machinery. but i knew better. it's the first time we ever had a use for 'forty-seven,' tom."

"and i hope it will be the last," replied the young inventor with a faint smile. "but i'd like to know what andy foger is doing in this neighborhood."

tom was soon himself again and able to go to the house, where he found mrs. baggert brewing a big basin of catnip tea, under the impression that it would in some way be good for him. she could not forgive herself for not having answered his signal, and as for mr. jackson, he had started for a doctor as soon as he learned that tom was shut up in the tank. the services of the medical man were canceled by telephone, as there was no need for him, and the engineer came back to the house.

tom was fully himself the next day, and aided his father and mr. sharp in putting the finishing touches to the advance. it was found that some alteration was required in the auxiliary propellers, and this, much to the regret of the young inventor, would necessitate postponing the trial a few days.

"but we'll have her in the water next friday," promised mr. swift.

"aren't you superstitious about friday?" asked the balloonist.

"not a bit of it," replied the aged inventor. "tom," he added, "i wish you would go in the house and get me the roll of blueprints you'll find on my desk."

as the lad neared the cottage he saw, standing in front of the place, a small automobile. a man had just descended from it, and it needed but a glance to show that he was mr. addison berg.

"ah, good morning, mr. swift," greeted mr. berg. "i wish to see your father, but as i don't wish to lay myself open to suspicions by entering the shop, perhaps you will ask him to step here."

"certainly," answered the lad, wondering why the agent had returned. getting the blueprints, and asking mr. berg to sit down on the porch, tom delivered the message.

"you come back with me, tom," said his father. "i want you to be a witness to what he says. i'm not going to get into trouble with these people."

mr. berg came to the point at once.

"mr. swift," he said, "i wish you would reconsider your determination not to enter the government trials. i'd like to see you compete. so would my firm."

"there is no use going over that again," replied the aged inventor. "i have another object in view now than trying for the government prize. what it is i can't say, but it may develop in time—if we are successful," and he looked at his son, smiling the while.

mr. berg tried to argue, but it was of no avail. then he changed his manner, and said:

"well, since you won't, you won't, i suppose. i'll go back and report to my firm. have you anything special to do this morning?" he went on to tom.

"well, i can always find something to keep me busy," replied the lad, "but as for anything special—"

"i thought perhaps you'd like to go for a trip in my auto," interrupted mr. berg. "i had asked a young man who is stopping at the same hotel where i am to accompany me, but he has unexpectedly left, and i don't like to go alone. his name was—let me see. i have a wretched memory for names, but it was something like roger or moger."

"foger!" cried tom. "was it andy foger?"

"yes, that was it. why, do you know him?" asked mr. berg in some surprise.

"i should say so," replied tom. "he was the cause of what might have resulted in something serious for me," and the lad explained about being imprisoned in the tank.

"you don't tell me!" cried mr. berg. "i had no idea he was that kind of a lad. you see, his father is one of the directors of the firm by whom i am employed. andy came from home to spend a few weeks at the seaside, and stopped at the same hotel that i did. he went off yesterday afternoon, and i haven't seen him since, though he promised to go for a ride with me. he must have come over here and entered your shop unobserved. i remember now he asked me where the submarine was being built that was going to compete with our firm's, and i told him. i didn't think he was that kind of a lad. well, since he's probably gone back home, perhaps you will come for a ride with me, tom."

"i'm afraid i can't go, thank you," answered the lad. "we are very busy getting our submarine in shape for a trial. but i can imagine why andy left so hurriedly. he probably learned that a doctor had been summoned for me, though, as it happened, i didn't need one. but andy probably got frightened at what he had done, and left. i'll make him more sorry, when i meet him."

"don't blame you a bit," commented mr. berg. "well, i must be getting back."

he hastened out to his auto, while tom and his father watched the agent.

"tom, never trust that man," advised the aged inventor solemnly.

"just what i was about to remark," said his son. "well, let's get back to work. queer that he should come here again, and it's queer about andy foger."

father and son returned to the machine shop, while mr. berg puffed away in his auto. a little later, tom having occasion to go to a building near the boundary line of the cottage property which his father had hired for the season, saw, through the hedge that bordered it, an automobile standing in the road. a second glance showed him that it was mr. berg's machine. something had gone wrong with it, and the agent had alighted to make an adjustment.

the young inventor was close to the man, though the latter was unaware of his presence.

"hang it all!" tom heard mr. berg exclaim to himself. "i wonder what they can be up to? they won't enter the government contests, and they won't say why. i believe they're up to some game, and i've got to find out what it is. i wonder if i couldn't use this foger chap?"

"he seems to have it in for this tom swift," mr. berg went on, still talking to himself, though not so low but that tom could hear him. "i think i'll try it. i'll get andy foger to sneak around and find out what the game is. he'll do it, i know."

by this time the auto was in working order again, and the agent took his seat and started off.

"so that's how matters lie, eh?" thought tom. "well, mr. berg, we'll be doubly on the lookout for you after this. as for andy foger, i think i'll make him wish he'd never locked me in that tank. so you expect to find out our 'game,' eh, mr. berg? well, when you do know it, i think it will astonish you. i only hope you don't learn what it is until we get at that sunken treasure, though."

but alas for tom's hopes. mr. berg did learn of the object of the treasure-seekers, and sought to defeat them, as we shall learn as our story proceeds.