orward compartment of the boat.

"The whole trick is rather easy," Jack informed Mr. Farnum, as they
walked that night in the village and discussed the matter in undertones.

"But you were in not more than seventy feet of water there," suggested
the builder. "You couldn't do it at much greater depth."

"At eighty feet of water I could do it," replied Benson, thoughtfully.

"But at a greater depth than eighty feet--?"

"Of course, the deeper one gets, the more tremendous the pressure of the
water is," answered the young captain. "At a depth of a hundred feet,
say, the pressure of the water would be enough to crowd me back into
the tube, crushing my body."

"And killing you," clicked Mr. Farnum.

"Undoubtedly. Yet seventy feet is as deep as one need go. Fifty feet
is far enough below the surface, for that matter. And we have the
splendid little 'Pollard' under such perfect control that we can drop to
fifty feet below the surface, as shown by our submersion gauge, and
keep just at that depth."

"It's all wonderful," cried the boatbuilder. "Jack, you are a genius
at this work!"

"There are some rather big problems to be worked out, in connection with
this new idea," hinted Benson.

"What are the problems?"

"Well, in observing a stretch of water, for the position or approach of
a hostile battleship, it might be necessary for the swimmer to go up
several times."

"Yes--?"

"That would call for a very considerable use of compressed air."

"Naturally."

"So, in the boat now building, Mr. Farnum, I think Mr. Pollard and
yourself should provide for the carrying of greater quantities of
compressed air. For, when a submarine is below, you must always have
reserve tanks of compressed air to be used in bringing the boat to the
surface. Of course, once on the surface, with the motor going, more
compressed air can be quickly stored."

"You've been doing some busy thinking, Jack," spoke Mr. Farnum,
approvingly.

"I haven't been doing it all, sir," was Benson's quick reply. "Hal and
Eph have been talking it all over with me."

"The Melvilles are very anxious to find out how you performed the
seemingly wonderful feat of leaving the submerged boat and then returning
to it."

"Are you going to tell them, sir."

"Not, at any rate, until I've taken more time to think about it. Yet,
you understand, Jack, I can't be too offish with them. They are able
to control the investment of a good deal of money, and