Jack crawling back into the forward 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.