"well, i guess they've had their lesson," remarked tom, as he took an observation through the telescope and saw andy and his cronies hard at work trying to repair the ruptured tires. "that certainly was a corking good trick."

"yes," admitted mr. sharp modestly. "i once did something similar, only it was a horse and wagon instead of an auto. but let's try for another speed record. the conditions are just right."

they arrived at the coast much sooner than they had dared to hope, the red cloud proving herself a veritable wonder.

the remainder of that day, and part of the next, was spent in working on the submarine.

"we'll launch her day after to-morrow," declared mr. swift enthusiastically. "then to see whether my calculations are right or wrong."

"it won't be your fault if it doesn't work," said his son. "you certainly have done your best."

"and so have you and mr. sharp and the others, for that matter. well, i have no doubt but that everything will be all right, tom."

"there!" exclaimed mr. sharp the next morning, as he was adjusting a certain gage. "i knew i'd forget something. that special brand of lubricating oil. i meant to bring it from shopton, and i didn't."

"maybe i can get it in atlantis," suggested tom, naming the coast city nearest to them. "i'll take a walk over. it isn't far."

"will you? i'll be glad to have you," resumed the balloonist. "a gallon will be all we'll need."

tom was soon on his way. he had to walk, as the roads were too poor to permit him to use the motor-cycle, and the airship attracted too much attention to use on a short trip. he was strolling along, when from the other side of a row of sand dunes, that lined the uncertain road to atlantis, he heard some one speaking. at first the tones were not distinct, but as the lad drew nearer to the voice he heard an exclamation.

"bless my gold-headed cane! i believe i'm lost. he said it was out this way somewhere, but i don't see anything of it. if i had that eradicate sampson here now i'd—bless my shoelaces i don't know what i would do to him."

"mr. damon! mr. damon!" cried tom. "is that you?"

"me? of course it's me! who else would it be?" answered the voice. "but who are you. why, bless my liver! if it isn't tom swift!" he cried. "oh, but i'm glad to see you! i was afraid i was shipwrecked! bless my gaiters, how are you, anyhow? how is your father? how is mr. sharp, and all the rest of them?"

"pretty well. and you?"

"me? oh, i'm all right; only a trifle nervous. i called at your house in shopton yesterday, and eradicate told me, as well as he could, where you were located. i had nothing to do, so i thought i'd take a run down here. but what's this i hear about you? are you going on a voyage?"

"yes."

"in the air? may i go along again? i certainly enjoyed my other trip in the red cloud. that is, all but the fire and being shot at. may i go?"

"we're going on a different sort of trip this time," said the youth.

"where?"

"under water."

"under water? bless my sponge bath! you don't mean it!"

"yes. dad has completed the submarine he was working on when we were off in the airship, and it will be launched the day after to-morrow."

"oh, that's so. i'd forgotten about it. he's going to try for the government prize, isn't he? but tell me more about it. bless my scarf-pin, but i'm glad i met you! going into town, i take it. well, i just came from there, but i'll walk back with you. do you think—is there any possibility—that i could go with you? of course, i don't want to crowd you, but—"

"oh, there'll be plenty of room," replied the young inventor. "in fact, more room than we had in the airship. we were talking only the other day about the possibility of you going with us, but we didn't think you'd risk it."

"risk it? bless my liver! of course i'll risk it! it can't be as bad as sailing in the air. you can't fall, that's certain."

"no; but maybe you can't rise," remarked tom grimly.

"oh, we won't think of that. of course, i'd like to go. i fully expected to be killed in the red cloud, but as i wasn't. i'm ready to take a chance in the water. on the whole, i think i prefer to be buried at sea, anyhow. now, then, will you take me?"

"i think i can safely promise," answered tom with a smile at his friend's enthusiasm.

the two were approaching the city, having walked along as they talked. there were still some sand dunes near the road, and they kept on the side of these, nearest the beach, where they could watch the breakers.

"but you haven't told me where you are going," went on mr. damon, after blessing a few dozen objects. "where do the government trials take place?"

"well," replied the lad, "to be frank with you, we have abandoned our intention of trying for the government prize."

"not going to try for it? bless my slippers! why not? isn't fifty thousand dollars worth striving for? and, with the kind of a submarine you say you have, you ought to be able to win."

"yes, probably we could win," admitted the young inventor, "but we are going to try for a better prize."

"a better one? i don't understand."

"sunken treasure," explained tom. "there's a ship sunk off the coast of uruguay, with three hundred thousand dollars in gold bullion aboard. dad and i are going to try to recover that in our submarine. we're going to start day after to-morrow, and, if you like, you may go along."

"go along! of course i'll go along!" cried the eccentric man. "but i never heard of such a thing. sunken treasure! three hundred thousand dollars in gold! my, what a lot of money! and to go after it in a submarine! it's as good as a story!"

"yes, we hope to recover all the treasure," said the lad. "we ought to be able to claim at least half of it."

"bless my pocketbook!" cried mr. damon, but tom did not hear him. at that instant his attention was attracted by seeing two men emerge from behind the sand dune near which he and mr. damon had halted momentarily, when the youth explained about the treasure. the man looked sharply at tom. a moment later the first man was joined by another, and at the sight of him our hero could not repress an exclamation of alarm. for the second man was none other than addison berg.

the latter glanced quickly at tom, and then, with a hasty word to his companion, the two swung around and made off in the opposite direction to that in which they had been walking.

"what's the matter?" asked mr. damon, seeing the young inventor was strangely affected.

"that—that man," stammered the lad.

"you don't mean to tell me that was one the happy harry gang, do you?"

"no. but one, or both of those men, may prove to be worse. that second man was addison berg, and he's agent for a firm of submarine boat builders who are rivals of dad's. berg has been trying to find out why we abandoned our intention of competing for the government prize."

"i hope you didn't tell him."

"i didn't intend to," replied tom, smiling grimly, "but i'm afraid i have, however. he certainly overheard what i said. i spoke too loud. yes, he must have heard me. that's why he hurried off so."

"possibly no harm is done. you didn't give the location of the sunken ship."

"no; but i guess from what i said it will be easy enough to find. well, if we're going to have a fight for the possession of that sunken gold, i'm ready for it. the advance is well equipped for a battle. i must tell dad of this. it's my fault."

"and partly mine, for asking you such leading questions in a public place," declared mr. damon. "bless my coat-tails, but i'm sorry! maybe, after all, those men were so interested in what they themselves were saying that they didn't understand what you said."

but if there had been any doubts on this score they would have been dissolved had tom and his friend been able to see the actions of mr. berg and his companion a little later. the plans of the treasure-hunters had been revealed to their ears.