they could hardly realize it, yet the depth-gage told the story. it registered a distance below the surface of the ocean of five thousand seven hundred feet—a little over a mile. the advance had actually come to rest on the bottom of the atlantic.

"hurrah!" cried tom. "let's get on the diving suits, dad, and walk about on land under water for a change."

"no," said mr. swift soberly. "we will hardly have time for that now. besides, the suits are not yet fitted with the automatic air-tanks, and we can't use them. there are still some things to do before we start on our treasure cruise. but i want to see how the plates are standing this pressure."

the advance was made with a triple hull, the spaces between the layers of plates being filled with a secret material, capable of withstanding enormous pressure, as were also the plates themselves. mr. swift, aided by mr. jackson and captain weston, made a thorough examination, and found that not a drop of water had leaked in, nor was there the least sign that any of the plates had given way under the terrific strain.

"she's as tight as a drum, if you will allow me to make that comparison," remarked captain weston modestly. "i couldn't ask for a dryer ship."

"well, let's take a look around by means the searchlight and the observation windows, and then we'll go back," suggested mr. swift. "it will take about two days to get the stores and provisions aboard and rig up the diving suits; then we will start for the sunken treasure."

there were several powerful searchlights on the advance, so arranged that the bow, stern or either side could be illuminated independently. there were also observation windows near each light.

in turn the powerful rays were cast first at the bow and then aft. in the gleams could be seen the sandy bed of the ocean, covered with shells of various kinds. great crabs walked around on their long, jointed legs, and tom saw some lobsters that would have brought joy to the heart of a fisherman.

"look at the big fish!" cried mr. damon suddenly, and he pointed to some dark, shadowy forms that swam up to the glass windows, evidently puzzled by the light.

"porpoises," declared captain weston briefly, "a whole school of them."

the fish seemed suddenly to multiply, and soon those in the submarine felt curious tremors running through the whole craft.

"the fish are rubbing up against it," cried tom. "they must think we came down here to allow them to scratch their backs on the steel plates."

for some time they remained on the bottom, watching the wonderful sight of the fishes that swam all about them.

"well, i think we may as well rise," announced mr. swift, after they had been on the bottom about an hour, moving here and there. "we didn't bring any provisions, and i'm getting hungry, though i don't know how the others of you feel about it."

"bless my dinner-plate, i could eat, too!" cried mr. damon. "go up, by all means. we'll get enough of under-water travel once we start for the treasure."

"send her up, tom," called his father. "i want to make a few notes on some needed changes and improvements."

tom entered the lower pilot house, and turned the valve that opened the tanks. he also pulled the lever that started the pumps, so that the water ballast would be more quickly emptied, as that would render the submarine buoyant, and she would quickly shoot to the surface. to the surprise of the lad, however, there followed no outrushing of the water. the advance remained stationary on the ocean bed. mr. swift looked up from his notes.

"didn't you hear me ask you to send her up, tom?" he inquired mildly.

"i did, dad, but something seems to be the matter," was the reply.

"matter? what do you mean?" and the aged inventor hastened to where his son and captain weston were at the wheels, valves and levers.

"why, the tanks won't empty, and the pumps don't seem to work."

"let me try," suggested mr. swift, and he pulled the various handles. there was no corresponding action of the machinery.

"that's odd," he remarked in a curious voice "perhaps something has gone wrong with the connections. go look in the engine-room, and ask mr. sharp if everything is all right there."

tom made a quick trip, returning to report that the dynamos, motors and gas engine were running perfectly.

"try to work the tank levers and pumps from the conning tower," suggested captain weston. "sometimes i've known the steam steering gear to play tricks like that."

tom hurried up the circular stairway into the tower. he pulled the levers and shifted the valves and wheels there. but there was no emptying of the water tanks. the weight and pressure of water in them still held the submarine on the bottom of the sea, more than a mile from the surface. the pumps in the engine-room were working at top speed, but there was evidently something wrong in the connections. mr. swift quickly came to this conclusion.

"we must repair it at once," he said. "tom, come to the engine-room. you and i, with mr. jackson and mr. sharp, will soon have it in shape again."

"is there any danger?" asked mr. damon in a perturbed voice. "bless my soul, it's unlucky to have an accident on our trial trip."

"oh, we must expect accidents," declared mr. swift with a smile. "this is nothing."

but it proved to be more difficult than he had imagined to re-establish the connection between the pumps and the tanks. the valves, too, had clogged or jammed, and as the pressure outside the ship was so great, the water would not run out of itself. it must be forced.

for an hour or more the inventor, his son and the others, worked away. they could accomplish nothing. tom looked anxiously at his parent when the latter paused in his efforts.

"don't worry," advised the aged inventor. "it's got to come right sooner or later."

just then mr. damon, who had been wandering about the ship, entered the engine-room.

"do you know," he said, "you ought to open a window, or something."

"why, what's the matter?" asked tom quickly, looking to see if the odd man was joking.

"well, of course i don't exactly mean a window," explained mr. damon, "but we need fresh air."

"fresh air!" there was a startled note in mr. swift's voice as he repeated the words.

"yes, i can hardly breathe in the living-room, and it's not much better here."

"why, there ought to be plenty of fresh air," went on the inventor. "it is renewed automatically."

tom jumped up and looked at an indicator. he uttered a startled cry.

"the air hasn't been changed in the last hour!" he exclaimed. "it is bad. there's not enough oxygen in it. i notice it, now that i've stopped working. the gage indicates it, too. the automatic air-changer must have stopped working. i'll fix it."

he hurried to the machine which was depended on to supply fresh air to the submarine.

"why, the air tanks are empty!" the young inventor cried. "we haven't any more air except what is in the ship now!"

"and we're rapidly breathing that up," added captain weston solemnly.

"can't you make more?" cried mr. damon. "i thought you said you could make oxygen aboard the ship."

"we can," answered mr. swift, "but i did not bring along a supply of the necessary chemicals. i did not think we would be submerged long enough for that. but there should have been enough in the reserve tank to last several days. how about it, tom?"

"it's all leaked out, or else it wasn't filled," was the despairing answer. "all the air we have is what's in the ship, and we can't make more."

the treasure-seekers looked at each other. it was an awful situation.

"then the only thing to do is to fix the machinery and rise to the surface," said mr. sharp simply. "we can have all the air we want, then."

"yes, but the machinery doesn't seem possible of being fixed," spoke tom in a low voice.

"we must do it!" cried his father.

they set to work again with fierce energy, laboring for their very lives. they all knew that they could not long remain in the ship without oxygen. nor could they desert it to go to the surface, for the moment they left the protection of the thick steel sides the terrible pressure of the water would kill them. nor were the diving suits available. they must stay in the craft and die a miserable death—unless the machinery could be repaired and the advance sent to the surface. the emergency expanding lifting tank was not yet in working order.

more frantically they toiled, trying every device that was suggested to the mechanical minds of tom, his father, mr. sharp or mr. jackson, to make the pumps work. but something was wrong. more and more foul grew the air. they were fairly gasping now. it was difficult to breathe, to say nothing of working, in that atmosphere. the thought of their terrible position was in the minds of all.

"oh, for one breath of fresh air!" cried mr. damon, who seemed to suffer more than any of the others. grim death was hovering around them, imprisoned as they were on the ocean's bed, over a mile from the surface.