Mary Nestor, as well as Tom Swift, felt great alarm over the condition of Mr. Keith. But the nurse, after reviving him, said:

"He is in no special immediate danger. Talking about his trouble overstrained him, but in the end it may do him good."

"Then will he get well?" asked Mary.

"He may," was the noncommittal answer. "His recovery would be hastened, however, if his mind could be relieved. He keeps worrying about the loss of his papers that proved his share in the Texas oil wells. Until they can be given back to him he is bound to suffer mentally, and of course that effects him physically."

"Oh, if we only could do something!" murmured Mary.

"Perhaps we can," said Tom in a low voice. "I've learned something these last few hours. I don't want to promise too much, but I think I begin to see how matters lie. There, he's rousing. Speak to him, Mary."

Mr. Keith opened his eyes, and smiled at his niece.

"Did I dream it," he asked in a low voice, "or was there some young man with you, Mary, my dear, to whom I was telling my troubles about the oil-well papers?"

"You didn't dream it, Uncle," Mary answered. "You were talking to Tom Swift. Here he is," and Tom came forward.

"Oh, yes, I remember now," said Mr. Keith passing his hand wearily over his eyes. "I thought, for a moment, that he had recovered my papers for me. But that was a dream, I'm sure."

"It may not be, Mr. Keith!" exclaimed Tom.

"May not be? What do you mean?"

"I mean," replied the young inventor, "that I am much interested in what you have told me. Now that I have proved that the Dixwell Hardley who is to sail with me is the same one who has treated you so shabbily, I think I understand the truth. I don't want to make a promise that I may not be able to carry out, but I am going to watch this man while he's on the submarine with me."

"Then you are going on with the voyage, Tom?" asked Mary.

"I shall have to," he said. "I have entered into an agreement with this man and I'm not going to break my contract, no matter what he does. But I think I know what his game is. Mr. Keith, I'm going to ask you to keep quiet about this matter until I come back from the treasure search. I may then have some news for you."

"I hope you do, young man, I hope you do!" exclaimed the oil contractor, with more energy than he had previously shown. "It means a lot, at my age, to lose a small fortune. If I were well and strong I'd tackle this Dixwell Hardley myself, and make him give up the papers I'm sure he has hidden away. He has them, I'm positive."

"Well, he may not have them, but perhaps he knows where they are," said Tom. "And I'm going to make it my business to watch him and see if I can find out his secret. I won't let him know I've heard from you. I'll apply the old saying of giving him plenty of rope, and I'll watch what happens.

"Now, Mr. Keith, take care of yourself. Mary and I must be getting back. Try not to worry, and I'll do my best for you," Tom concluded.

Mary added a few words of comfort and encouragement to her uncle, and then she and Tom took leave of him, flying back to Shopton in the speedy Air Scout.

"What are you going to do, Tom?" asked Mary, as he left her at her home, having told Mr. and Mrs. Nestor his part in the visit to Barton Keith.

"I'm going to start on the submarine voyage tomorrow," was the answer of the young inventor.

"Do you really believe there is a treasure ship?"

"Well, I've satisfied myself that a ship named the Pandora sunk about where Hardley says it did, and she had some treasure on board. Whether it's just the kind he has told me it was I don't know. But I'm going to find out."

"Then you'll be saying goodbye for a long time," observed Mary, rather wistfully.

"Oh, it may not be for so very long," and Tom tried to speak cheerfully. "I'll bring you back some souvenirs from the bottom of the sea," he added with a laugh.

"Bring me back--yourself!" said Mary in a low voice, and then she hurried away.

By appointment Tom met Mr. Damon and Mr. Hardley at the submarine dock the next morning. Everything had been made ready for the start, postponed from the day before. Mr. Hardley's estimated share of the expenses had been deposited in a bank, to be paid over later.

"Well, are we really going this time, or are you going to delay again?" asked the gold seeker, and his voice lacked a pleasant tone.

"Oh, were going this time!" exclaimed Tom. "And I hope everything turns out the way I want it to," he added meaningly.

"We'll find the treasure on the ship all right, if we can find the ship," said Mr. Hardley. "That part is your job, Mr. Swift."

"And I'll find her if she's where you say she went down," answered Tom. "Now then, as soon as Ned comes we'll start."

Ned Newton had been intrusted with some last-moment messages, but he arrived a little later, and hurried on board the M. N. 1 which lay at her dock, just afloat.

"All aboard!" called Tom, when he saw his financial manager coming down the pier. "We're ready to start now."

"Bless my fountain pen!" exclaimed Mr. Damon, "but we ought to do something, Tom--sing a song, make a speech or something, oughtn't we

"We'll sing a song of victory when we come back," replied Tom, with a laugh. "Everything all right at home, Ned?" he asked, for his chum had just come on from Shopton.

"Yes; your father sent his regards, but he told me to make a last appeal to you to install a gyro-scope rudder."

"It's too late for that now," said Tom. "He attaches, I think, too much importance to that device. I shan't need it with the improvements I have made to the craft. Get aboard!"

Ned climbed down the hatchway, which, however, was not closed, as it was decided to navigate the craft on the surface until it was necessary to submerge her because of too rough water, or when the vicinity of the wreck was reached.

"Though we will go down to the bottom when we get to the Atlantic for the purpose of testing her in deep water," decided Tom. "Most of the time we'll steam on the surface, for we'll save our batteries that way, and it's more comfortable breathing natural air."

So, with part of her deck above the surface, the M. N. 1 began her voyage, sent on her way by the cheers of the small force of Tom's workmen at the submarine plant. The general public was not admitted, for the object of the quest was kept secret from all save those immediately interested.

"Rad, him be plenty mad he not come," said Koku to Tom, as the giant moved about the cabin, putting things to rights.

"Well, don't start crowing over him until we get back," warned the young inventor. "He may have the laugh on us."

"Rad no laugh," declared Koku. "Rad him too mad dat I come on trip."

"A submarine voyage is no place for old, faithful Eradicate," murmured Tom. "He's better off looking after my father."

The first part of the trip was without incident of moment. No mishap attended the voyage of the M. N. 1 down the river, out into the bay, and so on to the great Atlantic.

Fairly good time was made, as there was no particular object in speeding, and on the second day after leaving the dock Tom gave orders for the hatch to be closed, the deck cleared, and everything made tight and fast.

"What's up?" asked Ned, hearing the instructions passed around.

"We're approaching deep water," was the answer. "I'm going to submerge."

A little later, by means of her diving rudders, aided also by the tanks, the M. N. 1 began to sink. Down, down, down she went.

"Now I'll be able to show you some pretty sights, Mr. Hardley," said Tom, as he and his friends entered the forward compartment, while the steel shutters were rolled back from the heavy glass windows. "We'll be in deep waters presently."

Ten minutes later the depth gauge showed that they were down about three hundred feet, and that is pretty deep for a submarine. But Tom's boat was capable of even greater depths than that.

At first there was nothing much to observe save the opal-tinted water illuminated by the powerful lights of the submarine. Small, and evidently frightened, fish darted to and fro, but there was nothing especially to attract the attention of Tom and his friends, who had made much more sensational trips than this under water.

Mr. Hardley, however, was fascinated, and kept close to the observation windows.

"Are there any wrecks around here?" he asked Tom.

"Possibly," was the answer. "Though they do not contain any treasure, I imagine--brick schooners or cargo boats would be about all."

The submarine went deeper, plowing her way through the Atlantic at a depth of more than three hundred and fifty feet, for Tom wanted to subject her to a good test.

Suddenly Mr. Hardley, who was now alone at the window on the port side, uttered a cry of alarm.

"Look! Look!" he fairly shouted. "We're surrounded by a school of sharks! What monsters! Are we in danger?"