“I suppose he opened the manhole cover, and rose to the surface,” hazarded Mr. Melville.

“In that case, sir,” smiled Captain Jack, “wouldn’t you expect the ‘Pollard’ to be filled with water, and my companions drowned?  Besides, sir, at a depth of seventy feet, the pressure of the water is such that it would be sheer impossibility to raise the manhole cover.”

“Then how did you get here?” demanded the capitalist.

“Pardon me, sir,” replied Jack, courteously, though firmly.

“Do you refuse to answer my question, boy?”

Again the irritating, half-contemptuous use of “boy” made Jack’s cheeks flush, though he answered merely: 

“I think, sir, Mr. Farnum has a right to the first information.”

“Do you understand, boy, that I am about to take a large interest in this business?”

“I have heard so, sir.  But I hope you won’t mind my saying that this little surprise was thought out by my comrades and myself.  It seems to me, therefore, that we have some rights in the disclosing of the secret.”

“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 easily.  “Bring the boat here.”

Clambering aboard, he directed the casting of the anchor overboard.  Then, poising himself at the bow, he made a strong dive, vanishing under the water.

“What’s he going to do now?” asked Mr. Melville, curiously.

“I’d rather wait than guess,” smiled Mr. Farnum.