"What is it, Tom? What is it?" cried Mr. Damon, not stopping in this moment of excitement to bless anything. "What is going to attack Ned and Koku?"

"I don't know," answered the young inventor. "It's some big fish evidently. I must get to the diving chamber!"

He gave a quick glance through the observation windows. Ned and the giant were moving as fast as they could toward the side of the craft where they could enter. The black, shadowy form was nearer now, but its nature could not be made out.

Calling to his force of assistants, Tom stood ready to let his chum and Koku out of the diving chamber as soon as the water should have been pumped from it.

A little later, as they all stood waiting in tense eagerness, there came a signal that the two divers had entered the side chamber. Quickly Tom turned the lever that closed the outer door.

"They're safe!" he exclaimed, as he started the pumps to working. But even as he spoke they felt a jar, and the submarine rolled partly over as if she had collided with some object. Yet this could not be, as she was stationary on the floor of the ocean.

"Bless my cake of soap, Tom!" cried Mr. Damon, "what in the world is that?"

"If it's an accident!" exclaimed Mr. Hardley, "I think it ought to be prevented. There have been too many happenings on this trip already. I thought you said your submarine was safe for underwater trips!" he fairly snapped at Tom.

The young inventor gave one look at the irate man who was coming out in his true colors. But it was no time to rebuke him. Too much yet remained to be done. Ned and Koku were still in the chamber and protected from some unknown sea monster by only a comparatively thin door. They must be inside to be perfectly safe.

Tom speeded up the pumps that were forcing the water from the chamber so the inner door could be opened. Eagerly he and his men watched the gauges to note when the last gallon should have been forced out by the compressed air. Not until then would it be safe to let Ned and Koku step into the interior of the craft.

The submarine had not ceased rolling from the force of the blow she had received when there came another, and this time on the opposite side. Once more she rolled to a dangerous angle.

"Bless my tea biscuit!" cried Mr. Damon, "what is it all about, Tom Swift?"

"I don't know," was the low-voiced answer, "unless a pair of monsters are attacking us on both sides alternately. But we'll soon learn. There goes the last of the water!"

The gauge showed that the diving chamber was empty. Quickly the inner doors were opened, stud, with their suits still dripping from their immersion in the salty sea, Ned and Koku stepped forth. In another moment their helmets were loosed from the bayonet catches, and they could speak.

"What was it, Ned?" cried Tom.

"Big fish!" answered Koku.

"Two monster whales!" gasped Ned. "We barely got away from them! They're ramming the sub, Tom!"

As he spoke there came a blow on the port side, greater than either of the two preceding ones. Those in the M. N. 1 staggered about, and had to hold on to objects to preserve their footing.

"Both at the same time!" cried Ned. "The two whales are coming at us both at once!"

This was evidently the case. Tom Swift quickly hurried to the engine room.

"What are you going to do?" asked Mr. Hardley. "You ought to do something! I'm not going to be killed down here by a whale. You've got to do something, Swift! I've had enough of this!"

Tom did not deign an answer, but hurried on. Mr. Damon followed him, having seen that some of the sailors were helping Ned and Koku out of the diving suits.

"Are we in any danger, Tom?" asked the eccentric man.

"Yes; but I think it is easily remedied," was the answer. "We'll go up to the surface. I don't believe the whales will follow us. Or, if they do, they can't do much damage when we are in motion. It's because we are stationary and they are moving that the blows seem so violent. Unless they collide head on with us, in the opposite direction to ours, we ought to be able to get clear of them. If they persist in following us--"

He paused as he pulled over the lever that would send the M. N. 1 to the surface.

"Well, what then?" asked Mr. Damon.

"Then we'll have to use some weapon, and I have several," finished the young inventor.

A few moments later the craft was in motion, not before, however, she was struck another blow, but only a glancing one.

"We're puzzling them!" cried Tom.

Having done all that was possible for the time being, Tom hurried to the observation chamber, followed by the ethers. There Tom switched on the powerful lights. For a moment nothing was to be seen but the swirling, green water. Then, suddenly, a great shape came into view of the glass windows, followed by another.

"Whales!" cried Tom Swift. "And the largest I've ever seen

It was true. Two immense specimens of the cetacean species were in front of the submarine, one on either bow, evidently much puzzled over the glaring lights. They were bow-heads, and immense creatures, and it would not take many blows from them to disable even a stouter craft than was the submarine.

But the motion of the undersea ship, the bright lights, and possibly the feel of her steel skin was evidently not to the liking of the sea monsters. One, indeed, came so close to the glass that he seemed about to try to break it, but, to the relief of all, he veered off, evidently not liking the look of what he saw.

Just once again, before the craft reached the surface, was there another blow, this time at the stern. But it was a parting tap, and none others followed.

"They've gone!" exclaimed Mr. Damon, as the whales vanished from the sight of those in the forward cabin.

"Have you any adequate protection against these monsters of the deep?" asked Mr. Hardley in a fault-finding voice. "I should think you would have taken precautions, Swift!"

He had dropped the formal "Mr." and seemed to treat Tom as an inferior.

"We have other protection than running away," said the young inventor quietly. "There are guns we can use, and, if the whales had been far enough away, I could have sent a small torpedo at them. Close by it would be dangerous to use that, as it would operate on us just as the depth bombs operated on the German submarines. However, I fancy we have nothing more to fear."

And Tom was right. When the surface was reached and the main hatch opened, the sea was calm and there was no sight of the whales. They evidently had had enough of their encounter with a steel fish, larger even than themselves.

"But they surely were monsters," said Ned, as he told of how he and Koku had sighted the animals; for a whale is an animal, and not a fish, though often mistakenly called one.

"Koku was for attacking them with his axe," went on Ned, "but I motioned to him to beat it. We wouldn't have stood a show against such creatures. They were on us before we noticed their coming, but I presume the big submarine attracted them away from us."

"It might have been the lights you carried that drew them," suggested Tom. "I am glad you came out of it so well."

Mr. Hardley seemed to recover some of his former manners, once the peril was passed, but his conduct had been a revelation to Mr. Damon.

"Tom," said the eccentric man in private to the young inventor, "I'm disgusted with that fellow. I don't see how I was ever bamboozled into taking up his offer."

"I don't, either," replied Tom frankly. "But we're in for it now. We've agreed to do certain things, and I'll carry out my end of the bargain. However, I won't put up with any of his nonsense. He's got to obey orders on this ship! I know more than he thinks I do!"

The next two days the M. N. 1 progressed along on the surface, and nothing of moment occurred. Then, as they neared southern waters, and Tom desired to make some observations of the character of the bottom, it was decided to submerge. Accordingly, one day the order was given.

Not until the gauge showed a hundred fathoms, or six hundred feet, did the craft cease descending, and then she came to rest on the bottom of the sea--a greater depth than she had yet attained on this voyage.

"How beautiful!" exclaimed Mr. Damon, when Tom turned on the lights and they looked out of the forward cabin windows. "How wonderful and beautiful!"

Well might he say that, for they were resting on pure white sand, and about them, growing on the bottom of this warm, tropical sea were great corals, purple and white, of wondrous shapes, waving plants like ferns and palms, and, amid it all, swam fish of queer shapes and beautiful colors.

"This is worth waiting for!" murmured Ned. "If only moving pictures of this could be taken in colors, it would create a sensation."

"Perhaps I may try that some day," said Tom with a smile. "But just now I have something else to do. Ned, are you game for another try in the diving dress? I want to see how it operates with a new air tank I've fitted on. Want to try?"

"Sure I'll go out," was the ready answer. "It's nicer walking around on this white sand than on the black mud where we saw the whales. You can see better, too."

A little later he and one of the sailors were outside the submarine, walking around in the diving dress, while Tom and the others watched through the glass windows. The new air tank seemed to be working well, for Ned, coming close to the window, signaled that he was very comfortable.

He walked around with the sailor, breaking off bits of odd- shaped coral to bring back to Tom. Suddenly, as those inside the craft looked out, they saw the sailor turn from Ned's side, and with a warning hand, point to something evidently approaching. The next instant a queer shape seemed to envelope Ned Newton, coming out from behind a ledge of weed-draped coral. And a cry went up from those in the submarine as Ned was seen to be enveloped in long, waving arms.

"An octopus!" cried Mr. Damon. "Bless my soul, Tom, an octopus has Ned!"

"No, it isn't that!" cried the young inventor hoarsely. "It's some other monster. It has only five arms--an octopus has eight! I've got to save Ned!"

And he hurried toward the diving chamber, while the others, in fascinated horror, looked at the diver who was in such strange peril.