"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—?"