Under the warm, tropical sun the submarine floated idly on the surface of the calm sea. She had risen from the depths, her hatches had been opened, and now the crew, the owner, and his guests were breathing free air. The men were taking advantage of the period above water to wash out some of their garments, hanging them on improvised lines stretched along the deck. For Tom Swift had said he would remain above the surface all day.

Some slight repairs were necessary to the electric motors, and they could be made only when the craft was on the open sea. This, too, would afford a chance to recharge the batteries and repair one of them.

For the time being the search under the sea for the treasure ship Pandora had been abandoned. But it was not given up entirely. As Tom had announced to Ned, a new theory would be worked out. So far, cruising about in the place where the fillibuster ship was supposed to have gone down had resulted in nothing.

Mr. Damon, who had been below, shaving, came up on deck to see Tom and Ned tossing into the water large pieces of cork taken from spare life preservers. Tom tossed his in from one side of the deck, and Ned from the other. Then, as the eccentric man listened, he heard Tom say:

"I think mine is going to beat yours, Ned!"

"Then you've got another guess coming," declared the young financial man. "Mine's going twice as fast as yours is now, though yours did start off better."

"Bless my beefsteak!" exclaimed Mr. Damon, "what's this, Tom Swift? I thought we came on a treasure-hunting expedition, and here I find you and Ned playing some childish game! I hope you aren't laying any wagers on it!" Mr. Damon did not approve of gambling in any form.

"No, we aren't doing that," laughed Tom, as he dropped another bit of cork into the ocean.

"We are trying to arrive at some valuable scientific facts, Mr. Damon."

"Scientific facts--that childish play?"

"It isn't play," said Tom, turning to remark to Ned: "I think we've settled it. The current has a decided twist to the north."

"Yes," agreed his chum. "You were right, Tom."

"If you don't mind explaining," began Mr. Damon, "I should like to know--"

"We're trying to determine the drift of the ocean currents in this locality," Tom said.

"So we'll know better where to look for the Pandora," added Ned.

"Oh, so you haven't given up the hunt, then?" asked the eccentric man.

"By no means!" exclaimed Tom. "It's this way, Mr. Damon. We went down at as nearly the exact spot where the treasure-ship was sunk as we could determine by means of calculations. She wasn't there, nor could we find her by going around in circles. Then it occurred to me, and to some of the others also, including Ned, that the ocean currents might have shifted the position of the craft after she had sunk. There are powerful currents in the ocean, as you know, the Gulf Stream being one and the Japan Current another. Now there may be smaller ones in these waters that would produce a local effect.

"So Ned and I have been dropping bits of cork of different shapes into the water and watching which way they drifted. Our conclusion is that the currents here have a decided set toward the north."

"And what does that indicate?" asked Mr. Damon.

"That we should have begun our search some distance north of the point where we actually did begin," answered Tom.

"How far north?" the eccentric man wanted to know.

"That's just what we have yet to ascertain," the young inventor replied. "So far our conclusions have been arrived at merely from surface data. Now we've got to go below."

"And play with bits of cork there?" asked Mr. Damon.

"No, we'll have to use something heavier than cork," Tom said. "We'll probably use weights, and see how far they move along the bottom in a given time. But we have established one thing, and I begin to have hopes now that we may locate the Pandora."

The remainder of the day was spent in various ways aboard the submarine, which continued to float idly on the waves.

It was toward evening, when the red, setting sun gave promise of a fair day on the morrow that the submarine's deck lookout approached Tom, and, waiting until he had the attention of the young inventor, reported:

"There is a smudge of smoke dead astern, sir."

"Is there?" exclaimed Tom. "Let me have the glasses."

He took them from the lookout and made a long and careful study of the slight, black smudge which was low down on the horizon.

"A steamer," decided Tom, "and coming on fast. We'll go below!" he added. "Please make ready," he said to the officer in charge.

"What's up, Tom?" asked Ned, as his chum gathered up the papers on which he had been figuring on an improvised table set under an awning on deck.

"Some craft is coming, and I'd just as soon she wouldn't sight us," was the answer.

"You mean she might interfere with our search for the treasure- ship?"

"Not exactly. But she might want to start a search on her own account, and there's no use of giving our presence away, or letting them guess at what might be right conclusions as to the location of the Pandora."

"But, Tom, no one knows of the wreck! At least, no one is supposed to but our party and--"

"Hardley. Exactly!" exclaimed Tom, as he saw his chum about to utter the name.

"And you think he is coming?"

"I shouldn't be a bit surprised. Anyhow, it's just as easy for us to submerge and let them do their own guessing. I was going down soon, anyhow, and another hour won't make any difference. Here, take a look, if you like."

Ned peered through the glasses, but his eyes not being trained in sea interpretation, as were Tom's, he could make out nothing but a black smudge, now larger and darker.

"It might be a cloud for all I can tell," he said, as he handed the binoculars back to Tom.

"Well, it's a steamer all right, and she's under forced draft, too, if I'm any judge. We'll go below before she sights us."

"Perhaps she has already," suggested Ned, as the crew began clearing the submarine's deck.

"No, we lie too low in the water for that. Well, now we can start our underwater observations of current trends."

It did not take long, once she started, for the M. N. 1 to go down. Just as the sun sank below the horizon, and while the smudge of smoke was becoming more distinct, the waves closed over the steel deck of the submarine. Half an hour later she was nearly a quarter of a mile below the surface, resting on the bottom of the sea again.

On this trip Tom did not go to any such depths as he did on his former voyage in the Advance. Not that the reconstructed submarine was not capable of it, for she was even stronger than when first built. But the wreck they were seeking did not lie in so great a depth of water, and there was no need of running useless risks.

"Well," remarked Ned, when they came to a stop, "I don't believe any one will find us here."

"Not an ordinary diver, at any rate," Tom agreed. "And after supper I'm going to have another go at the currents."

The meal was served as usual, and a very good one it was, considering the fact that not as many supplies could be carried in the rather limited space of a submarine as may be transported in an ocean liner. Then, as it was still early, Tom and Ned, with the help of some of the officers, got ready for a new series of experiments.

The big searchlight was set aglow, and, going out on the ocean bed in diving suits, Tom and his friends dropped on the sand various weighted objects.

These were made in the shape of the hull of a steamer, and in proportion. Once they were on the sand, an iron rod was thrust into the ocean bed near each object.

"Now," remarked Tom, as they all went into the submarine again, "we'll let them drift until morning. Then we'll make new calculations. I think we'll arrive at some results, too."

"Just what are you aiming to do?" asked Mr. Damon.

"See how far each one of those weighted objects drifts," Tom replied. "We have planted them in different spots on the ocean bed. Some will drift farther than others. Some are large and some are small. By striking an average we may be able to tell about how far from the supposed location of the Pandora we ought to look for her."

The night passed without incident and as calmly and peacefully as though they were all in some deep cave beneath a great mountain. In the morning after breakfast Tom and his friends went outside the submarine again and noted the weighted objects. Some had drifted farther than others. Measurements were carefully taken, and then began a series of intricate calculations.

The distance each object had drifted from the iron bar marker was considered in reference to its size and shape. Also the elapsed time was computed. The results were then compared, an average struck, and then the size and weight of the Pandora, as nearly as they could be ascertained, were figured. The resultant figures were compared, and Tom announced:

"If we are anywhere near right in our conclusions we ought to begin to search for the treasure-ship about four miles from here, in a general northerly direction."

"Do you think she has drifted that far?" asked Ned.

"Fully that," Tom answered. "That is only our starting point-- the center of a new series of circles."

A moment later Tom gave the order to rise to the surface.

"Going up?" exclaimed Ned.

"Yes, I want to make some observations to determine our exact nautical position."

"But suppose that other steam--"

"We'll have to take a chance. We can submerge quickly if we have to, and I don't believe she's able to do that."

An observation was taken through the conning tower, however, before the M. N. 1 went all the way up, and there was not a sail nor a smudge of smoke on the horizon.

"So far so good," murmured Tom. "Now we'll 'shoot the sun,' and after we submerge we'll begin our search in earnest. I think we are on the right track now."

The observation was made at noon, and then, as nearly as possible, the submarine was moved to a position approximately four miles north of the place where the Pandora was supposed to have foundered.

"Down we go!" exclaimed Tom, and down they went.

The depth gauge showed more than a thousand feet below the surface when the M. N. 1 came to rest. This was deeper than Tom had thought to find the wreck, but his craft was able to withstand the pressure. A brief wait, to make sure that everything was in readiness, was followed by the beginning of the new search. In gradually widening circles the craft moved about under water.

If the voyagers had expected to locate at once the treasure- ship, they would have been disappointed. For the first day gave no signs. But Tom had not promised immediate results, and no one gave up hope.

It was shortly after noon on the second day of the search at the new location that, as they were proceeding at rather greater speed than usual, something happened.

Ned had just suggested that he and Tom might go out and try the current-setting experiments again, when suddenly they were both thrown off their feet by a terrific jar and concussion. The M. N. 1 seemed to reel back, as if from a great blow.

"Bless my safety razor!" cried Mr. Damon, "what's the matter, Tom?"

"I think we've had a collision!" was the answer. "I must see how badly we are damaged!"