At first Harry refused absolutely to consent to Dick's accompanying him, but after a long argument he was forced to yield.

"Why should you take all the risks when it isn't your own country, especially?" asked Dick, almost sobbing. "I've got a right to go! And, besides, you may need me."

That was true enough, as Harry realized. Moreover, he had been investigating the Bleriot, and he discovered that it was one of a new safety type, with a gyroscope device to insure stability. The day was almost without wind, and therefore it seemed that if such an excursion could ever be safe, this was the time. He consented in the end, and later he was to be thankful that he had.

Once the decision was taken, they waited impatiently for the return of Jack Young. Harry foresaw protests from Jack when he found out what they meant to do, but for him there was an easy answer—there was room in the aeroplane for only two people, and there was no way of carrying an extra passenger.

It was nearly dusk when Jack returned, and he had the forethought to bring a basket of food with him—cold chicken, bread and butter, and milk, as well as some fruit.

"I didn't find out very much," he said, "except this. Someone from London has been asking about you both. And this much more—at least a dozen people have come down to Bray Park to-day from London."

"Did you see any sign of soldiers from London?"

"No," said Jack.

He was disappointed when he found out what they meant to do, but he took his disappointment pluckily when he saw that there was no help for it. Harry explained very quietly to both Jack and Dick what he meant to do and they listened, open mouthed, with wonder.

"You'll have your part to play, Jack," said Harry. "Somehow I can't believe that the letter I wrote to Colonel Throckmorton last night won't have some effect. You have got to scout around in case anyone comes and tell them all I've told you. You understand thoroughly, do you?"

"Yes," said Jack, quietly. "When are you going to start?"

"There's no use going up much before eleven o'clock," said Harry. "Before that we'd be seen, and, besides, if a Zeppelin is coming, it wouldn't be until after that. My plan is to scout to the east and try to pick her up and watch her descend. I think I know just about where she'll land—the only place where there's room for her. And then—"

He stopped, and the others nodded, grimly.

"I imagine she'll have about a hundred and twenty miles to travel in a straight line—perhaps a little less," said Harry. "She can make that in about two hours, or less. And she'll travel without lights, and in the dark. Big as they are, those airships are painted so that they're almost invisible from below. So if she comes by night, getting here won't be as hard a job as it seems at first thought."

Then the three of them went over in every detail the plan Harry had formed. Dick and Harry 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 had 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 had 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 in 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 Zeppelin—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.

But he had been prepared, and was flying away. Down below there was now a great glare from the burning wreckage, lighting up the whole scene. And suddenly there was a sharp breaking out of rifle fire. At first he thought the men below had seen them, and were firing upward. But in a moment he saw the truth. Bray Park had been attacked from outside!

Even before they reached the ground, in the meadow where Harry and Jack had emerged from the tunnel, the firing was over. But now a search-light was playing on the ground on the opposite bank, and Harry and Dick saw, to their wonder and delight, that the ground swarmed with khaki-clad soldiers. In the same moment Jack ran up to them.

"The soldiers had the place surrounded!" he cried, exultingly. "They must have believed your letter after all, Harry! Come on—there's a boat here! Aren't you coming over?"

They were rowing for the other shore before the words were well spoken. And, once over, they were seized at once by two soldiers.

"More of them," said one of the soldiers. "Where's the colonel?"

Without trying to explain, they let themselves be taken to where Colonel Throckmorton stood near the burning wreckage. At the sight of Harry his face lighted up.

"What do you know about this?" he asked, sternly, pointing to the wrecked airship.

Harry explained in a few words.

"Very good," said the colonel. "You are under arrest—you broke arrest this morning. I suppose you know that is a serious offence, whether your original arrest was justified or not?"

"I felt I had to do it, sir," said Harry. He had caught the glint of a smile in the colonel's eyes.

"Explain yourself, sir," said the colonel. "Report fully as to your movements to-day. Perhaps I shall recommend you for a medal instead of court martialling you, after all."

And so the story came out, and Harry learned that the colonel had never believed Graves, but had chosen to let him think he did.

"The boy Graves is a German, and older than he seems," said the colonel. "He was here as a spy. He is in custody now, and you have broken up a dangerous raid and a still more dangerous system of espionage. If you hadn't come along with your aeroplane, we would never have stopped the raid. I had ordered aviators to be here, but it is plain that something has gone wrong. You have done more than well. I shall see to it that your services are properly recognized. And now be off with you, and get some sleep. You may report to me the day after to-morrow!"