"That's evidence enough," remarked Dick, "but to make assurance doubly sure we'll go over to where the aviator fell and you'll see that he was a German all right." The body had been decently covered up before the boys reached there, but the clothing and the effects found proved beyond a doubt that the aviator had been one of their foes. "Take it all back, Dick," said Frank. "You knew what you were about. And I'm glad that you came out of the scrap safe and sound. But it certainly was some scrap while it lasted." "It sure was," replied Dick. "That fellow was as skilful and plucky as they make them. He kept my hands full, and there was one time when he came within an ace of raking me. But luck was with me. Poor fellow! I'm sorry for him, but I'd have been still more sorry if it had been myself." "What beats me is the way you tumbled to him," puzzled Billy. "You surely couldn't have read the German markings under their coat of paint. How did you know he was a German?" Dick smiled. "Simple enough," he answered. "We Allied aviators have a secret system of signals, something like Freemasonry. When we come near another plane that seems to be one of our own, we make a certain dip of our plane. That's like asking for the countersign. If the other fellow's all right he makes a certain signal in return. If he doesn't do it the first time, we try again, because there's always a chance that he hasn't noticed our signal, or is too busy in handling his plane to give the reply. But if after two or three times we don't get the countersign, we know the fellow's a Hun and we open up on him." "Good stuff!" approved Billy. "That's what happened this morning," continued Dick. "This fellow came sailing along as calm and cheeky as you please, and was having a bully time taking pictures of our positions. At least I suppose that is what he was doing, as he evidently wasn't out looking for fight. I thought it wouldn't do any harm to take a look at him, although I saw the machine had French markings. I gave the signal, but of course he couldn't give the countersign. I repeated it three times without getting an answer, and then I pitched into him. That makes the thirteenth that I've brought down." "Thirteen was an unlucky number for him, all right," remarked Billy. "How are you fellows getting along?" asked Dick, stretching himself out on the ground for a brief resting spell. "I notice that you've been right up to your neck in fighting lately." "Its been pretty hot along this sector," Frank admitted, "though I suppose it's nothing to what it will be after the big German drive gets started. That is if it ever does start. I sometimes think they've given up the idea."