ONCE Tom Swift had made up his mind to do a thing he did it-- even though it was against his better judgment. His word, passed, was his bond.

In conformity then with his decision to take Mr. Damon and the latter's friend, Mr. Hardley, on an undersea search for treasure, Tom at once proceeded to make his preparations. Ned, too, had his work to do, since the decision to make what might be a long trip would necessitate a change in Tom's plans. But, as in everything he did, he threw himself into this whole-heartedly and with enthusiasm.

Not once did Tom Swift admit to himself that he was going into this scheme because he thought well of it. It was all for Mr. Damon, after Tom had learned that his friend had invested considerable money in a company Mr. Hardley had formed to pay half the expenses of the trip.

Tom even tried to buy Mr. Damon off, by offering the latter back all the money the eccentric man had invested with his new friend. But Mr. Damon exclaimed:

"Bless my gasolene tank, Tom! I'm in this thing as much for the love of adventure, as I am for the money. Now let's go on with it. You will like Hardley better when you know him better."

"Perhaps," said Tom dryly, but he did not think so.

The young inventor insisted, before making any preparations for the trip, that all the cards be laid on the table. That is, he wanted to be sure there had been such a ship as the Pandora, that she was laden with gold, and that she had sunk where Mr. Hardley said she had. The latter was perfectly willing to supply all needful proofs, even though some were difficult, because of the nature of the voyage of the treasure craft. As a filibuster she was not trading openly.

"Here are all the records," said Mr. Hardley to Tom one day, when the young inventor, Ned, and Mr. Damon were gathered in Tom's office. "You may satisfy yourself."

And, with Ned's help, Tom did.

There was no question but what the Pandora had sailed from a certain port on a certain date. The official reports proved that. And that she did carry a considerable treasure in gold was also established to the satisfaction of Tom Swift. Because the gold was to be used for furthering ends against one of the South American governments, the gold shipment was not insured and, in consequence, no recovery could be made.

"Then you are satisfied, are you, Mr. Swift, that the ship, set out with over two millions in gold on board?" asked Mr. Hardley. "Yes, that seems to be proved," Tom admitted, and Ned nodded. "The next thing to prove is that she foundered in a storm about the position I am going to tell you," went on Mr. Damon's friend.

"He doesn't tell you the exact location now, Tom," explained Mr. Damon, "because it might leak out. He'll disclose it to us as soon as we are out of sight of land in the submarine."

"I'm willing to agree to that proposition," Tom said. "But I want to be sure she really did sink."

This was proved to him by official records. There was no question but that the Pandora had gone down in a big storm. And Mr. Hardley was on board. He proved that, too, a not very difficult task, since the official passenger list was open to inspection.

Mr. Hardley repeated his story about having overheard the exact location of the ship a few minutes before she sank, and he also told of the captain and several members of the ship's company having been drowned. This, too, was confirmed.

"Then," went on Mr. Hardley, "all that remains for me to do is to deposit at some bank my half of the expenses and await your word to go aboard the submarine."

"I believe that is all," returned Tom. "But, on my part, it will take some little time to fit the submarine out as I want to have her. There are some special appliances I want to take along which will aid us in the search for the gold, if we find the place where the Pandora is sunk."

"Oh, we'll find that all right," declared Mr. Hardley, "if you will only follow my directions."

Tom looked slightly incredulous, but said nothing.

Then followed busy days. The submarine Advance, which had made several successful trips, as related in the book bearing the title, "Tom Swift and His Submarine Boat," was hauled into dry dock and the work of overhauling her begun. Tom put his best men to work, and, after a consultation with his father, decided on some radical changes in the craft.

"Tom, my boy," said the aged Mr. Swift, "I wish you weren't going on this trip."

"Why, Dad?" asked the young inventor.

"Because I fear something will happen. We don't really need this money, and suppose--suppose--"

"Oh, I'm not worrying, Dad," was the answer. "I've taken worse risks than this, many a time. I'm really doing it as a favor to Mr. Damon. He's got too much money invested to let him lose it. And we can use a million dollars ourselves. It will enable me to put in operation a plan to pension our workmen. I've long had that in mind, but I've never had enough capital to carry it out."

"Well, of course, Tom, that's a worthy object, and I won't make any further objections. But take my advice, and strengthen the submarine."

"Why, Dad?" asked Tom in some surprise. "Because you'll find the water there of a greater depth than you think," was the answer. "I know you have the official hydrographic charts, but there's a mistake, I'm sure. I once made a study of that part of the ocean, and there are currents there at certain seasons of the year that no one suspects, and deep caverns that aren't charted. If the Pandora lies in one of these you'll need a great strength of walls to your submarine to withstand the pressure of deep water."

The craft Tom Swift proposed to use in searching for the treasure ship Pandora was of the regular cigar-shape, but inside it had many special features. It was more comfortable than the usual submarine, not being intended for fighting, though it did carry guns and a torpedo tube. Tom intended renaming the craft, which had been called Advance, and one day, when there had been some discussion as to what the undersea craft ought to be called, Ned explained:

"Why don't you name it after her?"

"After whom?" inquired Tom, in some surprise, looking up from a letter he was writing.

"Your friend and future wife, Mary Nestor," answered Ned. "I'm sure she'd appreciate it."

"That isn't such a bad idea," conceded Tom musingly. "The only thing about it is that I don't want Mary's name bandied about that way."

"Use her initials, then," suggested Ned.

"How do you mean

"Why not call it the M. N. 1.? Isn't that a good name?"

"The M. N. 1." mused Tom. "Not so bad. If the N. C. 4 flew over the ocean the M. N. 1 ought to be able to navigate under it. I think I'll do that, Ned."

So the Advance, rebuilt and refitted in many ways, was christened the M. N. 1, and a wonderful craft she proved to be. Mary Nestor was quite pleased when Tom told her what he had done. She appreciated the delicate compliment he had paid her.

Busy and more busy were the days that passed. As the M. N. 1 had to be refitted some miles from Tom's home, where it was feasible to launch her for the trip, he had to make the journey between the drydock and his shop either by automobile or aeroplane. Often he choose the latter, since he had a number of small, speedy craft in his hangars. Sometimes Ned or Mr. Damon went with him, but Mr. Hardley could never be induced to ride in an airship.

"I'll travel on the ocean or under it," he said, "but I'm not going to take a chance in the air. I'm too afraid of falling."

"Tom, what's this?" asked Ned one day, when he and Tom had come to see how the work of remodeling the submarine was getting along. "It looks like something you used when you dug your big tunnel."

"That's a new kind of diving bell," Tom answered. "You know it isn't easy to get treasure out of a sunken ship. It isn't like picking it off the bottom of the ocean. We've got to get it out from inside--perhaps from inside a strong box or a safe. This bell may come in useful."

"Can't you use the special diving suits that you always used to carry?" the financial manager wanted to know.

"We might, if the water isn't too deep," replied Tom. "But you know there is a limit to how far down a man in even my kind of diving dress can go. With this diving bell a much greater depth can be reached. And this diving bell is not like any you have ever seen or read about. My father gave me the idea for it. I'll demonstrate it to you some day."

A diving bell is shaped like its name. A common glass tumbler thrust down into a pail of water, with the open side down, will show exactly the principle on which a diving bell works. It illustrates the fact that two things cannot occupy the same place at the same time.

Pushing the tumbler, open end down, into the pail of water, leaves a space in the upper end of the tumbler which the water cannot fill, because it is already occupied with air. Imagine a big tumbler, made of thick steel, lowered into the water. Air pumped into the upper part not only keeps the water from entering, but also enables a man inside to breathe and to move about inside the bell which may be lowered to the floor of the ocean. But, as Tom told Ned, his diving bell was a big improvement over those commonly used.

The two young men inspected the progress made in refitting the submarine, and Tom expressed himself as satisfied.

"How soon do you think you can start?" asked Ned.

"In about two weeks," was the answer. "I'll want to get to the West Indies before the fall storms start. Not only will it be impossible to make a search then, but the very location of the sunken wreck may be changed."

"How so?" asked Ned.

"Because of undersea currents. They are strong enough, not only to sweep a wreck away from the place where it may have settled, but they may cover it with sand, and then it is hopeless to try to dig it out. So We've got to go soon, if we go at all."

"Well, I'm with you!" exclaimed Ned. "Hello! here's some one looking for you, I guess," he added, as a boy came hurrying down to the dock from the temporary office Tom had set up there.

"You're wanted on the telephone, Mr. Swift," said the messenger. "It's important, too."

"All right. I'll come at once," was the answer. "Hope it isn't bad news," mused Ned, as his chum hurried on in advance. "Maybe Hardley has found out he hasn't a right to search for that sunken gold after all. That would be too bad for Mr. Damon!"