"WHAT'S that noise?" asked Mr. Hardley.

Mr. Hardley, Tom Swift, Mr. Damon, Ned Newton, Koku, and one or two navigating officers of the craft, were gathered in the operating cabin of the M. N. 1.

"That's water being pumped into the tanks," explained Tom. "We are now going down. If you'll watch the depth gauge you can note our progress."

"Going down, are we?" remarked Mr. Hardley. "Well, it's interesting to say the least," and he observed the gauge, which showed them to be twenty feet under the surface.

"Bless my hydrometer, but he's got nerve for a first trip in a submarine! He's all right, isn't he?" whispered Mr. Damon to Tom.

"Well, I'm glad to see he isn't nervous," remarked Tom, honest enough to give his visitor credit for what was due him. And indeed many a person is nervous going down in a submarine for the first time. "Still we can't go more than thirty feet down in this water," went on Tom. "A better test will be when we get about five hundred feet below the surface. That's a real test, though as far as knowing it is concerned, a person can't tell ten feet from ten hundred in a submarine under water, unless he watches the gauge."

"Well, I think you'll find Mr. Hardley all right," said Mr. Damon, who seemed to have taken a strong liking to his new friend.

Certainly the latter showed no signs of nervousness as the craft slowly settled to the proper depth. He asked numberless questions, showing his interest in the operation of the M. N. 1, but he showed not the least sign of fear. However, as Tom said, that might come later.

"We are going down now," Tom explained, as he pointed out to Mr. Hardley the various controlling wheels and levers, "by filling our ballast tanks with water. We can rise, when needful, by forcing out this water by means of compressed air. When we are on the ocean we can go down by using our diving rudders, and in much quicker time than by filling our tanks."

"How is that?" asked the seeker after the Pandora's gold.

"Filling the tanks is slow work in itself," replied Tom, "and they have to be filled very carefully and evenly, so we don't stand on our stern or bow in going down. We want to sink on an even keel, and sometimes this is hard to accomplish. But we are doing it now," and he called attention to an indicator which told how much the M. N. 1 might be listing to one side or to one end or the other.

"A submarine, as everyone knows, is essentially a water-tight tank, shaped like a cigar, with a propeller on one end. It can sink below the surface and move along under water. It sinks because rudders force it down, and water taken into tanks in its interior hold it to a certain depth. It can rise by ejecting this extra water and by setting the rudders in the proper position.

A submarine moves under water by means of electric motors, the current of which is supplied by storage batteries. On the surface when the hatches can be opened, oil or gasolene engines are used. These engines cannot be used under water because they depend on a supply of air, or oxygen, and when the submarine is tightly sealed all the air possible is needed for her crew to breathe. While cruising on the surface a submarine recharges her storage batteries to give her motive power when she is submerged.

There are many types of submarines, some comparatively simple and small, and others large and complex. In some it is possible for the crew to live many days without coming to the surface.

Tom Swift's reconstructed craft compared favorably with the best and largest ever made, though she was not of exceptional size. She was very strong, however, to allow her to go to a great depth, for the farther down one goes below the surface of the sea, the greater the pressure until, at, say, six miles, the greatest known depth of the ocean, the pressure is beyond belief. And yet is possible that marine monsters may live in that pressure which would flatten out a block of solid steel into a sheet as thin as paper.

"Well, we are as deep down as it is safe to go in the river," announced Tom, as the gauge showed a distance below the surface of a little less than twenty-nine feet. "Now we'll move into the bay. How do you like it, Mr. Hardley?"

"Very well, so far. But it isn't very exciting yet."

"Bless my accident policy!" exclaimed Mr. Damon, "I hope you aren't looking for excitement."

"I'm used to it," was the answer. "The more there is the better I like it."

"Well, you may get your wish," said Tom.

He turned a lever, and those on board the submarine became conscious of a forward motion. She was no longer sinking.

She trembled and vibrated as the powerful electric motors turned her propellers, and Tom, having seen that all was running smoothly in the main engine room, called Mr. Damon, Ned, and Mr. Hardley to him.

"We'll go into the forward pilot house and give Mr. Hardley a view under water," he announced. "Of course, you'll see nothing like what you'll view when we're in the ocean," added the young inventor, "but it may interest you."

The four were soon in the forward compartment of the craft. She could be directed and steered from here when occasion arose, but now Tom was letting his navigator direct the craft from the controls in the main engine room. A conning tower, rising just above the deck of the craft, gave the pilot the necessary view.

"Here you are!" exclaimed Tom, as he switched out the lights in the cabin. For a moment they were in darkness, and then, with a click, steel plates, guarding heavy plate glass bull's-eyes, moved back, and Mr. Hardley for the first time looked out on an underwater scene. He saw the murky waters of river down which they were proceeding to the bay moving past the glass windows. Now and then a fish swam up, looking in, and, with a swirl of its tail, shot away again, apparently frightened well-nigh to death.

"Bless my shoe laces, Tom!" exclaimed Mr. Damon, "this isn't a marker compared to some of the sights we've seen, is it?"

"I can imagine not," said Mr. Hardley. "But it is interesting. I shall be anticipating more wonderful sights."

"And you'll get them!" exclaimed Ned. "Do you remember, Tom, the time the big octopus tried to hold us back?"

"Yes, indeed," answered the young inventor. "That gave us a scare for the time being."

Steadily the M. N. 1 kept on her way under water. Her path was illuminated to a considerable degree by a broad, diffused beam of light from a powerful searchlight that was fixed just back of the conning tower, giving the helmsman a certain degree of vision. This light also served to illuminate the water, so that those in the forward cabin could see what was going on around them.

"There isn't much of interest in the river," said Tom. "No big fish, or anything else of moment. Even in the bay we won't see much to attract our attention. But I want to make sure everything is working smoothly before we start for the West Indies."

"That's right!" agreed Mr. Hardley. "We want to make a success of this trip."

He remained at the glass bull's-eyes, now and then exclaiming as some shad or other fair-sized fish came into view. Suddenly, however, his exclamation was sharper than usual.

"Look!" he exclaimed. "There's part of a wreck!"

Ned, Mr. Damon, and Tom looked out and saw, sweeping past them, the ribs and worm-eaten timbers of some craft, lying on the bottom of the river.

"Yes, that's the remains of an old brick scow," the young inventor explained. "That's one of our water-marks, so to speak. It is at the bend of the river. We turn now, and head for the bay."

As he spoke they all became aware of a sudden swerve in the course of the submarine. The helmsman had, doubtless, noted the "water-mark," as Tom termed it, and as an automobilist on land might swing at the cross-roads, the steersman was changing the course of his craft.

"We'll go deeper," said Tom a moment later, as the wreck passed out of view. "We can go about fifty feet down now. Yes, he's sinking her," he added, as a gauge showed the craft to be descending. "Nelson knows his business all right."

"He is your captain?" asked Mr. Hardley.

"One of the best, yes. He'll go with us on the search for the Pandora."

They talked of various matters, Tom relating to Mr. Hardley how a tug had rammed the brick scow some years ago, and sunk it in the river.

The submarine was now about forty-eight feet below the surface, and suddenly they all became aware that her speed had increased.

"Guess he's going to give the motors a good try-out," observed Tom. "I think I'll go back to the engine room. You may remain here, if you like, and you'll probably see--"

A cry from Mr. Damon interrupted him.

"Bless my rubber boots, Tom! Look!" cried the eccentric man. "We're going to ram a mud bank!"

As he spoke they all became aware of a solid black mass looming in front of the bull's-eye window. An instant later the submarine came to a jarring stop, as if she had struck some soft, yielding substance. There was a confused shouting throughout the craft, the noise of machinery, a trembling and vibration, and then ominous quiet.