"Humph!" broke in Don Melville. "It's all some deception--some cheap
trick, anyway."

Captain Jack held up one hand to signal the shore boat, which, with two
workmen in it, was hovering near. As the boat came in, the submarine
boy announced:

"Now, I will show you the rest of the principle that my mates and I are
demonstrating. Mr. Farnum, by the way, has just spoken of the humane
side of this discovery, the making possible the rescue of a crew of a
boat that can't be made to rise. Gentlemen, there's still another side
to it. Under actual war conditions, with a submarine boat guarding a
coast or harbor entrance, if the commander of the boat brought the
conning tower above the surface, the presence of the boat would be
detected on a clear day. But the head of a swimmer rising from the
boat could not be observed at any very great distance. Yet the swimmer
could make out the hull or masts of a hostile vessel some miles away.
This new trick is likely to make submarine boats much more valuable to
the countries owning them. Now, I want to try something else, and see
whether I can do it."

The shore boat put in when called. In the bow was a hundred-pound
anchor, with plenty of cable to pay out after it. Captain Jack entered
the boat, looked over the anchor tackle, then returned to shore.

"Come to me where I stop," he directed the men in the boat. With that,
after getting his bearings fully, he swam out, counting his strokes as
he went.

"It's about here that I came up," he called, pausing and treading water