Nearing the former school-house that had been pointed out to them, the two chums took their bearings afresh. Crossing the road one at a time, with utmost stealth, they reached the other side without having been challenged. A little further on they espied a German sentry, pacing post. Waiting until the fellow had gone to the furthest limit of his post, the chums, flat on their stomachs, crawled forward until, on looking backward, they judged it safe to rise and move on crouchingly. Then they came in sight of the aviation station. "Better crawl all the way now," Dick whispered. "We have reached the point where any attempt at speed will be sure to place a few bullets in our bodies." Tom nodded, without speaking. It was trampled, withered grass through which they now crawled. It offered fair concealment, but there was danger of making a noise that might betray them to a keen-eared sentry. At last, near the first hangar, they reached a spot where two trees stood close together. Crawling to this shelter, they still remained lying down, though the tree trunks gave them greater safety against being seen. In front of the hangars paced a sentry; at the rear another soldier walked post. At some distance from this latter sentry stood four tents, in which, Papa Prim had declared, slept the reliefs of the guard. "I see how we could get the sentry at the rear," Dick whispered, after a few minutes' silent survey. "But it's at the front that we want to get in, and I don't see any way of creeping up on the front sentry without the rear sentry seeing us and firing. That would give the alarm." "Then we've got to 'get' the rear sentry first?" Tom asked, his lips at his chum's ear. "That's it." "Nasty business, and double chance of losing the game." "It's the only way, Tom, unless your head is working better than mine." For some minutes Tom Reade studied. "I guess it will have to be the rear sentry first," he conceded. At that moment a small door at the rear of the hangar opened. The two friends heard the noise, and judged by sound more than sight. "Sentry!" said the man who had stepped outside, in a low voice. "Herr Lieutenant!" responded the man. "I am not locking the door, sentry. I shall be back before long." "Very good, Herr Lieutenant." Passing to the front of the hangar this German aviation lieutenant waited until the sentry there had reached him, then delivered the same information, after which the aviation officer strode off briskly toward the troop camp that could be only vaguely seen in the distance. "It sounds as if he intended to make a flight," whispered Dick uneasily. "That wouldn't be so bad," Reade replied. "It will be worse if his machine is out of order and he is coming back to fuss over it." "We must make our break now," Prescott whispered.