"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."