Then the three of them went over in every detail the plan Harry had formed.  Dick and Jack took their places in the monoplane and rehearsed every movement they would have to make.

“I can’t think of anything else that we can provide for now,” said Harry, at last.  “Of course, we can’t tell what will come up, and it would be wonderful if everything came out just as we have planned.  But we’ve provided for everything we can think of.  You know where you are to be, Jack?”

“Yes.”

“Then you’d better start pretty soon.  Good-bye, Jack!” He held out his hand.  “We could never have worked this out without you.  If we succeed you’ll have a big part in what we’ve done.”

A little later Jack said good-bye in earnest, and then there was nothing to do but wait.  About them the voices of the insects and frogs changed, with the darkening night.  The stars came out, but the night was a dark one.  Harry looked at his watch from time to time and at last he got up.

“Time to start!” he said.

He felt a thrill of nervousness as the monoplane rose into the air.  After all, there was a difference between being the pilot and sitting still in the car.  But he managed very well, after a few anxious moments in the ascent.  And once they were clear of the trees and climbing swiftly, in great spirals, there was a glorious sensation of freedom.  Dick caught his breath at first, then he got used to the queer motion, and cried aloud in his delight.

Harry headed straight into the east when he felt that he was high enough.  And suddenly he gave a cry.

“Look!” he shouted in Dick’s ear.  “We didn’t start a moment too soon.  See her — that great big cigar-shaped thing, dropping over there?”

It was the Zepplin — the battleship of the air.  She was dipping down, descending gracefully, over Bray Park.

“I was right!” cried Harry.  “Now we can go to work at once — we won’t have to land and wait!”

He rose still higher, then flew straight for Bray Park.  They were high, but, far below, with lights moving about her, they could see the huge bulk of the airship, as long as a moderate sized ocean liner.  She presented a perfect target.

“Now!” said Harry.

And at once Dick began dropping projectiles they had found in the aeroplane — sharply pointed shells of steel.  Harry had examined these —­ he found they were really solid steel shot, cast like modern rifle bullets, and calculated to penetrate, even without explosive action, when dropped from a height.

From the first two that Dick dropped there was no result.  But with the falling of the third a hissing sound came from below, and as Dick rapidly dropped three more, the noise increased.  And they could see the lights flying — plainly the men were running from the monster.  Its bulk lessened as the gas escaped from the great bag and then, in a moment more, there was a terrific explosion that rocked the monoplane violently.  Had Harry not been ready for it, they might have been brought down.