"THIS is my busy day!" announced the young inventor as he went into the Nestor sitting room, where the telephone was installed.

"Perhaps it is some one else who wants you to come to their rescue," suggested Mary.

But it was not, as Tom related a little later when he had finished his talk over the wire.

"Just a business matter," he announced to Mary and her mother, when he rejoined them. "A gentleman with whom I expect to make a submarine trip is at the house, and wants to consult with me about details. He is getting anxious to start. Mr. Damon is there, too."

"Blessing every thing he lays eyes on, I suppose," remarked Mrs. Nestor, with a smile.

"Yes, and some things he doesn't see," agreed Tom. "He is going with us on this submarine trip."

"Oh, Tom, are you going to undertake another of those dangerous voyages?" asked Mary, in some alarm.

"Well, I don't know that they are particularly dangerous," replied Tom, with a smile. "But we expect to make a search for a sunken treasure ship in a submarine. That's the vessel I'm working on now," he added. "We're rebuilding the Advance, you know, making her more up-to-date, and adding some new features, including her name--M. N. 1."

"I suppose Mr. Damon's friend is getting anxious to make a start, particularly as he has already invested several thousand dollars in the project," went on the young inventor. "He formed a company to pay half the expenses of the search, and they will share in the~ treasure--if we find it," Tom said. "I wish Mr. Damon, who holds most of the shares the promoter let out of his own hands, had not gone into it, but, since he has, I'm going to do the best I can for him."

"Then aren't you friendly with the other man?" asked Mary.

"I don't especially care for him," the young inventor admitted. "He isn't just my style--too fond of himself, and all that. Still I may be misjudging him. However, I'm in the game now, and I'm going to stick. I'll have to be traveling on," he said. "Mr. Damon and his friend are at my house, and they've been telephoning all over to find me. I guess this was one of the first places they tried," he said with a smile, referring to the fact that he spent considerable time at Mary's home.

"Well, I'm glad they found you, but I'm sorry you have to go," Mary said with a smile.

A little later Tom Swift, with Ned, for whom he called, was on his way back home in his Air Scout, having said goodbye to Mary and her mother and expressing the hope that Mr. Keith would soon be over his business troubles.

"Oil wells are queer, anyhow," mused Tom.

Then Tom got to thinking about Dixwell Hardley: "I don't like the man, and the more I see of him the less I like him. But I'm in for it now, and I'll stick to the finish. I only wish I could locate the treasure ship, give him his share, and get back to my work. I'm going to try to turn out an airship that a man can use as handily as he does a flivver now."

Musing on the possibilities in this field, Tom, having left Ned at the latter's home, soared down from aloft, and a little later, having told Koku to look after the Air Scout, much to the delight of the giant and the discomfiture of Rad, the young inventor was closeted with Mr. Damon and Dixwell Hardley.

"Bless my straw hat, Tom!" exclaimed the eccentric man, "but we just couldn't wait any longer. How are you coming on, and when can we start on this treasure-hunting trip? I declare it makes me feel young again to think about it!"

"Well, it won't be long now," was the answer. "The men are working hard to get the submarine in shape, and I should say that in another week, or two weeks at the most, we could set off!"

"Good!" exclaimed Mr. Hardley. "I have received additional information," he went on, "to the effect that the amount of gold on board the Pandora was even greater than we at first thought."

"That sounds encouraging," replied Tom. "It only remains to find the sunken ship now. But what interests me greatly is whether, after we have gotten this gold, supposing we are successful, we shall be allowed to keep it."

"Bless my bank book! why not?" asked Mr. Damon. "Isn't it wealth abandoned at the bottom of the sea, and isn't finding keeping?"

"Not always," answered Tom. "There are certain rules and laws about treasure, and it might happen that after we got this--if we do--it could be taken away from us."

"I think there will be no difficulty on this score," said Mr. Hardley. "In the first place, two attempts were made to get this wealth, and were unsuccessful. Then it was practically abandoned, and I believe under the law the persons who now find it will be entitled to keep it. Besides the persons who gathered it together did so for an unlawful purpose--that of starting a revolution in a friendly country--and they would not dare claim it for fear of giving their secret away."

"Well, perhaps you are right," assented Tom. "We'll make a try for it, anyhow."

"You say the submarine is nearly ready?" asked Mr. Hardley.

"She will be ready for a trial trip at the end of this week," said Tom, "and be fitted up for the voyage within another seven days, I hope. Then for the great adventure!" and he laughed, though, truth to tell, he had no real liking for his task. The more he saw of Mr. Hardley the less he liked him.

"I shall begin getting my affairs in shape," said the latter, as he gathered up some papers he had brought to attempt to prove to Tom that the wealth of the Pandora was greater than had been supposed. "I have many large interests," he went on, rather pompously, "and they need looking after; especially if I undertake anything so extra hazardous as a submarine trip."

"Yes, there always is some danger," admitted Tom. "But then there is danger walking along the street."

"Oh, there's no danger with Tom Swift!" exclaimed Mr. Damon. "I've been under the sea and above the clouds with him, and, bless my rainbow! he always brought us safe home."

"And I'll try to do the same this time," said the young inventor.

Busy days followed for Tom Swift and his friends. The force at work on the submarine turned night into day to rush her completion, and in due season she was set afloat in the dry dock basin and formally rechristened the M. N. 1.

Mary blushed as she gave the boat her new name, and there was a little cheer from the group of workmen gathered at the dock. There was no launching in the real sense of the word, since as the Advance that ceremony had been gone through with for the undersea craft.

She had been greatly changed interiorly and outwardly. Her skin, or plates, having been doubled and strengthened. For Tom proposed to go to a much greater depth than ever before.

In addition to using the submarine herself in a search for the gold on the Pandora, Tom had installed on board some new kinds of diving apparatus and also a diving bell. If one would not serve, the other might, he reasoned.

"Well, Tom," remarked his aged father the night before they were to start on the trial trip, "I understand you have practically rebuilt the Advance."

"Yes; and I think she's a much better craft, too, Father."

"Glad to hear that, Tom. Of course you kept the gyroscope rudder feature?"

"No, I didn't," replied Tom. "If I had left that installed it would have meant carrying a smaller diving bell, and I think that last will be more useful than the gyroscope. I put in a set of double-acting depth rudders instead."

Mr. Swift shook his head.

"I'm sorry for that, Tom," he remarked. "There's nothing like the gyroscope rudder in a tight pinch--say when there's a storm. And for holding the boat steady, if you have to make a sudden turn under water, to avoid an obstruction you come upon unexpectedly, a gyroscope can't be improved on. It holds you steady and prevents your turning turtle."

"I've put side fin-keels to correct that," Tom explained.

But still his father was not satisfied.

"I'd rather you had kept the gyroscope," he said, and the time was to come when Tom Swift wished that himself.

But it was too late to make the change now, and so, with more than usual confidence in his own designing abilities, the next day the young inventor and his friends went aboard the M. N. 1 for the trial trip.

"You don't easily get seasick, do you?" Tom asked Mr. Hardley, as they descended the hatchway into the interior of the craft.

"No, I'm considered a good sailor."

"Well, you'll need to be," went on Tom, with a smile. "Not that we are likely to strike any rough water now, though the reports say a stiff breeze is blowing in the bay. But when we once start for the West Indies you are likely to experience a new sensation. I've known sailors who never had any qualms, even in terrible storms, to get ill in a submarine when she went through only a small blow. The motion is different from that on a surface boat."

"I can imagine so," returned Mr. Hardley. "But I'll be thinking of the millions in gold on the Pandora, and that will keep my mind off being seasick."

"Let us hope so," murmured Tom.

He gave the word, they all descended, the hatch covers were closed down, and the M. N. 1 was ready to start on a trial trip.