Chapter Thirteen. Off for the Treasure
 

Suddenly Tom, after a moment's pause, seized a wrench 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 gasping.

"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 inert heap on the floor, with Captain Weston trying to force a few drops of stimulant 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 devoid 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 clogged. 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 afterward, 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 machinery 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 hissing 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 conning tower and let in fresh air as soon as the top was above the surface.

With a bound the Advance reached the top. Tom frantically 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 secluded creek where she had been launched, her passengers had fully recovered from their terrible experience, though the nerves of Mr. Swift and Mr. Damon were not at ease for some days thereafter.

"I should never have made a submerged test without making sure that we had a reserve supply of air," remarked the aged inventor. "I will not be caught that way again. But I can't understand how the pump valve got out of order."

"Maybe some one tampered with it," suggested Mr. Damon. "Could Andy Foger, any of the Happy Harry gang, or the rival gold-seekers have done it?"

"I hardly think so," answered Tom. "The place has been too carefully guarded since Berg and Andy once sneaked in. I think it was just an accident, but I have thought of a plan whereby such accidents can be avoided in the future. It needs a simple device."

"Better patent it," suggested Mr. Sharp with a smile.

"Maybe I will," replied the young inventor. "But not now. We haven't time, if we intend to get fitted out for our trip."

"No; I should say the sooner we started the better," remarked Captain Weston. "That is, if you don't mind me speaking about it," he added gently, and the others smiled, for his diffident comments were only a matter of habit

The first act of the adventurers, after tying the submarine at the dock, was to proceed with the loading of the food and supplies. Tom and Mr. Damon looked to this, while Mr. Swift and Mr. Sharp made some necessary changes to the machinery. The next day the young inventor attached his device to the pump valve, and the loading of the craft was continued.

All was in readiness for the gold-seeking expedition a week later. Captain Weston had carefully charted the route they were to follow, and it was decided to move along on the surface for the first day, so as to get well out to sea before submerging the craft. Then it would sink below the surface, and run along under the water until the wreck was reached, rising at times, as needed, to renew the air supply.

With sufficient stores and provisions aboard to last several months, if necessary, though they did not expect to be gone more than sixty days at most, the adventurers arose early one morning and went down to the dock. Mr. Jackson was not to accompany them. He did not care about a submarine trip, he said, and Mr. Swift desired him to remain at the seaside cottage and guard the shops, which contained much valuable machinery. The airship was also left there.

"Well, are we all ready?" asked Mr. Swift of the little party of gold-seekers, as they were about to enter the conning tower hatchway of the submarine.

"All ready, dad," responded his son.

"Then let's get aboard," proposed Captain Weston. "But first let me take an observation."

He swept the horizon with his telescope, and Tom noticed that the sailor kept it fixed on one particular spot for some time.

"Did you see anything?" asked the lad.

"Well, there is a boat lying off there," was the answer. "And some one is observing us through a glass. But I don't believe it matters. Probably they're only trying to see what sort of an odd fish we are."

"All aboard, then," ordered Mr. Swift, and they went into the submarine. Tom and his father, with Captain Weston, remained in the conning tower. The signal was given, the electricity flowed into the forward and aft plates, and the Advance shot ahead on the surface.

The sailor raised his telescope once more and peered through a window in the tower. He uttered an exclamation.

"What's the matter?" asked Tom.

"That other ship--a small steamer--is weighing anchor and seems to be heading this way," was the reply.

"Maybe it's some one hired by Berg to follow us and trace our movements," suggested Tom.

"If it is we'll fool them," added his father. "Just keep an eye on them, captain, and I think we can show them a trick or two in a few minutes."

Faster shot the Advance through the water. She had started on her way to get the gold from the sunken wreck, but already enemies were on the trail of the adventurers, for the ship the sailor had noticed was steaming after them.