Suddenly Tom, after a moment's pause, seized a wrench¹ and began loosening some nuts.  
"What are you doing?" asked his father faintly, for he was being weakened by the vitiated atmosphere.
 
"I'm going to take this valve apart," replied his son. "We haven't looked there for the trouble. Maybe it's out of order."
 
He attacked the valve with energy, but his hands soon lagged. The lack of oxygen was telling on him. He could no longer work quickly.
 
"I'll help," murmured Mr. Sharp thickly. He took a wrench, but no sooner had he loosened one nut than he toppled over. "I'm all in," he murmured feebly.
 
"Is he dead?" cried Mr. Damon, himself gasping².
 
"No, only fainted. But he soon will be dead, and so will all of us, if we don't get fresh air," remarked Captain Weston. "Lie down on the floor, every one. There is a little fairly good air there. It's heavier than the air we've breathed, and we can exist on it for a little longer. Poor Sharp was so used to breathing the rarified air of high altitudes that he can't stand this heavy atmosphere."
 
Mr. Damon was gasping worse than ever, and so was Mr. Swift. The balloonist lay an inert³ heap on the floor, with Captain Weston trying to force a few drops of stimulant⁴ down his throat.
 
With a fierce determination in his heart, but with fingers that almost refused to do his bidding, Tom once more sought to open the big valve. He felt sure the trouble was located there, as they had tried to locate it in every other place without avail.
 
"I'll help," said Mr. Jackson in a whisper. He, too, was hardly able to move.
 
More and more devoid⁵ of oxygen grew the air. It gave Tom a sense as if his head was filled, and ready to burst with every breath he drew. Still he struggled to loosen the nuts. There were but four more now, and he took off three while Mr. Jackson removed one. The young inventor lifted off the valve cover, though it felt like a ton weight to him. He gave a glance inside.
 
"Here's the trouble!" he murmured. "The valve's clogged⁶. No wonder it wouldn't work. The pumps couldn't force the water out."
 
It was the work of only a minute to adjust the valve. Then Tom and the engineer managed to get the cover back on.
 
How they inserted the bolts and screwed the nuts in place they never could remember clearly afterward⁸, but they managed it somehow, with shaking, trembling hands and eyes that grew more and more dim.
 
"Now start the pumps!" cried Tom faintly. "The tanks will be emptied, and we can get to the surface."
 
Mr. Sharp was still unconscious, nor was Mr. Swift able to help. He lay with his eyes closed. Garret Jackson, however, managed to crawl to the engine-room, and soon the clank of machinery⁹ told Tom that the pumps were in motion. The lad staggered to the pilot house and threw the levers over. An instant later there was the hissing¹⁰ of water as it rushed from the ballast tanks. The submarine shivered, as though disliking to leave the bottom of the sea, and then slowly rose. As the pumps worked more rapidly, and the sea was sent from the tank in great volumes, the boat fairly shot to the surface. Tom was ready to open the conning¹¹ tower and let in fresh air as soon as the top was above the surface.
 
With a bound the Advance reached the top. Tom frantically¹² worked the worm gear that opened the tower. In rushed the fresh, life-giving air, and the treasure-hunters filled their lungs with it.
 
And it was only just in time, for Mr. Sharp was almost gone. He quickly revived, as did the others, when they could breathe as much as they wished of the glorious oxygen.
 
"That was a close call," commented Mr. Swift. "We'll not go below again until I have provided for all emergencies. I should have seen to the air tanks and the expanding one before going below. We'll sail home on the surface now."
 
The submarine was put about and headed for her dock. On the way she passed a small steamer, and the passengers looked down in wonder at the strange craft.
 
When the Advance reached the secluded¹³ creek¹⁴ where she had be............