Tom Swift, who had been making readings of the various gauges, taking notes for future use, and otherwise busying himself about the navigation of his reconstructed craft, turned quickly from the instrument board at the cry from Mr. Hardley. The gold- seeker, with a look of terror on his face, had recoiled from the observation windows.

"Bless my hat band!" cried Mr. Damon. "Look, Tom!"

They all turned their attention to the glass, and through the plates could be seen a school of giant fishes that seemed to be swimming in front of the submarine, keeping pace with it as though waiting for a chance to enter.

"Are we well protected against sharks, Mr. Swift?" demanded the adventurer. "Are these sea monsters likely to break, the glass and get in at us?"

"Indeed not!" laughed Tom. "There is absolutely no danger from these fish--they aren't sharks, either."

"Not sharks?" cried Mr. Hardley. "What are they, then?"

"Horse mackerel," Tom answered. "At least that is the common name for the big fish. But they are far from being sharks, and we are in no danger from them."

"Oh!" exclaimed Mr. Hardley, and he seemed a little ashamed of the exhibition of fear he had manifested. "Well, they certainly seem determined to follow us," he added.

The big fish were, indeed, following the submarine, and it required no exertion on their part to maintain their speed, since below the surface the M. N. 1 could not move very fast, as indeed no submarine can, due to the resistance of the water.

"They do look as though they'd like to take a bite or two out of us," observed Ned. "Are they dangerous, Tom?"

"Not as a rule," was the answer. "I don't doubt, though, but if a lone swimmer got in a school of horse mackerel he'd be badly bitten. In fact, some years ago, when there was a shark scare along the New Jersey coast, some fishermen declared that it was horse mackerel that were responsible for the death and injury of several bathers. A number of horse mackerel were caught and exhibited as sharks, but, as you can easily see, their mouths lack the under-shot arrangement of the shark, and they are not built at all as are the man-eaters."

"Bless my toothbrush!" exclaimed Mr. Damon. "Still, between a horse mackerel and a shark there isn't much choice!"

Mr. Hardley, with a shudder, turned away from the glass windows, and Tom glanced significantly at Ned. It was another exhibition of the man's lack of nerve.

"We'll have trouble with him before this voyage is over," declared the young inventor to his chum, a little later.

"What makes you think so?" asked Ned.

"Because he's yellow; that's why. I thought him that once before, and then I revised my opinion. Now I'm back where I started. You watch--we'll have trouble."

"Well, I guess we can handle him," observed the financial manager.

"I'm going a little deeper," announced Tom, toward evening on the first day of the voyage on the open ocean. "I want to see how she stands the pressure at five hundred feet. I feel certain she will, and even at a greater depth. But if there's anything wrong we want to correct it before we get too far away from home. We're going down again, deeper than before."

A little later the submarine began the descent into the lower ocean depths. From three hundred and fifty feet she went to four hundred, and when the hand on the gauge showed four hundred and fifty there was a tense moment. If anything went wrong now there would be serious trouble.

But Tom Swift and his men had done their work well. The M. N. 1 stood the strain, and when the gauge showed four hundred and ninety feet Mr. Damon gave a faint cheer.

"Bless my apple dumpling, Tom!" he replied, "this is wonderful."

"Oh, we've been deeper than this," replied the young inventor, "but under different conditions. I'm glad to see how well she is standing it, though."

Suddenly, as the needle pointer on the depth gauge showed five hundred and two feet, there came a slight jar and vibration that was felt throughout the craft.

"What's that?" suddenly and nervously cried Mr. Hardley. "Have we struck something?"

"Yes, the bottom of the ocean," answered Tom quietly. "We are now on the floor of the Atlantic, though several hundred miles, and perhaps a thousand, from the treasure ship. We bumped the bottom, that's all," and as he spoke he brought the submarine to a stop by a signal to the engine room.

And there, as calmly and easily as some of the masses of seaweed growing on the ocean floor around her, rested the M. N. 1. It was a test of her powers, and well had she stood the test, though harder ones were in store for her.

And inside the submarine Tom and his party were under scarcely greater discomfort than they would have been on the surface. True, they were confined to a restricted space, and the air they breathed came from compression tanks, and not from the open sky. The lights had to be kept aglow, of course, for it was pitch dark at that depth. The sunlight cannot penetrate to more than a hundred feet. But sunlight was not needed, for the craft carried powerful electric lights that could illuminate the sea in the immediate vicinity of the submarine.

"Are you going to stay here long?" asked Mr. Hardley, when Tom had spent some time making accurate readings of the various instruments of the boat. "Of course, I realize that you are the commander, but if we don't get to the treasure ship soon some one else may loot her before we have a chance. She's been given up as a hopeless task more than once, but the lure of the millions may attract another gang."

"I want to stay here until I make sure that nothing is leaking and that everything is all right," answered the young inventor. "This is a test I have not given her since the rebuilding. But I think she is coming through it all right, and we can soon start off again. Before we do, though, I want to try the new diving outfit. Ned, are you game for it now? This is a little deeper than you have gone out in for some time, but--"

"Oh, I'm game!" exclaimed the young financial manager. "Get out the suit, Tom, and I'll put it on. I'll go for a stroll on the bottom of the sea. Who knows? Perhaps I may pick up a pearl."

"Pearls aren't found in these northern waters, any more than are sharks," said Tom with a laugh. "However, I'll have the suits made ready. I'll send Koku with you, and I'll stay in this time. Mr. Damon, do you want to go out?"

"Not this time, Tom," answered the eccentric man. "My heart action isn't what it used to be. The doctor said I mustn't strain it. At a depth not quite so great I may take a chance."

"How about you, Mr. Hardley?" asked Tom. "Do you want to put on one of my portable diving suits and walk around on the bottom of the sea?"

"I--I don't believe I've had enough experience," was the hesitating answer. "I'll watch the others first."

Tom felt that it would be this way, but he said nothing. He ordered the diving suits made ready, a special size having been built for the giant, and soon preparations were under way for the two to step outside the craft.

Those who have read of Tom Swift's submarine boat know how his special diving outfit was operated. Instead of the diver being supplied with the air through a hose connected with a pump on the surface, there was attached to the suit a tank of compressed air, which was supplied as needed through special reducing valves.

The diving dress, too, was exceptionally strong, to withstand the awful pressure of water at more than five hundred feet below the surface. The usual rubber was supplemented by thin, reinforced sheets of steel, and this feature, together with an auxiliary air pressure, kept the wearer safe.

Thus Ned and Koku could leave the submarine, walk about on the floor of the ocean as they pleased, and return, unhampered by an air hose or life line. In dangerous waters, infested by sea monsters, weapons could be carried that were effective under water. The diving suit was also provided with a powerful electric light operated by a new form of storage current, compact and lasting.

"Well, I think we're all ready," announced Ned, as he and Koku were helped into their suits and they waited for the glass- windowed helmets to be put on. Once these were fastened in place talk would have to be carried on with the outside world by means of small telephones or by signals.

"Give me axe!" exclaimed Koku, as some of the sailors were about to put his helmet in place.

"What do you want of an axe?" Tom asked.

"Maybe so one them cow fish come along," explained the giant. "Koku whack him with axe."

"He means horse mackerel," laughed Ned. "Give him the axe, Tom. I don't like the looks of those fish, either. I'll take a weapon myself."

Two keen axes were handed to the divers, their helmets were screwed on, and they immediately began breathing the compressed air carried in a tank on their shoulders.

Slowly and laboriously they walked to the diving chamber. Their progress would be easier in the water, which would buoy them up in a measure. Now they were heavily weighted.

To leave the submarine the divers had to enter a steel chamber in the side of the craft. This craft contained double doors. Once the divers were inside the door leading to the interior of the submarine was hermetically closed. Water from outside was then admitted until the pressure was equalized. Then the outer door was opened and Ned and Koku could step forth.

They entered the chamber, the door was closed tightly and then Tom Swift turned the valve that admitted the sea water. With a hiss the Atlantic began rushing in, and in a short time the outer door would be opened.

"If you'll come around to the observation windows you can see them," said Tom, when a look at the indicators told him Ned and Koku had stepped forth.

To the front cabin he and the others betook themselves, and when the interior lights were turned out and the exterior ones turned on they waited for a sight of the two divers.

"Bless my pickle bottle!" cried Mr. Damon, "there they are, Tom."

As he spoke there came into view, moving slowly, Ned and Koku. Their portable lights were glowing, and then, in order to see them better, Tom turned out the exterior searchlights. This made the two forms, in their rather grotesque dress, stand out in bold relief amid the swirling green waters of the Atlantic.

Ned and the giant moved slowly, for it was impossible to progress with any speed wader that terrific pressure. They looked toward the submarine and waved their hands in greeting. They had no special object on the ocean floor, except to try the new diving dress, and it seemed to operate successfully. Ned made a pretense of looking for treasure amid the sand and seaweed, and once he caught and held up by its tail a queer turtle. Koku stalked about behind Ned, looking to right and left, possibly for a sight of some monster "cow fish."

"They're coming back in, I think," remarked Tom, when he saw Ned turn and start back for the side of the craft, where, amidships, was located the diving chamber. "They're satisfied with the test."

Suddenly Koku was seen to glide to the side of Ned, and point at something which none of the observers in the M. N. 1 could see. The giant was evidently perturbed, and Ned, too, showed some agitation.

"Bless my rubber shoes! what's the matter?" cried Mr. Damon.

"I don't know," answered Tom. "Perhaps they have sighted a wreck, or something like that."

"Look! It's a sea monster!" cried Mr. Hardley. "I can see the form of some great fish, or something. Look! It's coming right at them!"

As he spoke all in the observation chamber saw a great, black form, as if of some monster, move close to the two divers.