"Yes," answered Jimmy, who had acquired considerable of this disquieting information. "Our side was planning a big raid, but not in this immediate neighborhood. On that account the headquarters staff sent for some of our reserves. They were taken off quietly enough, and it was thought the Germans wouldn't get wise to the fact. But they did, and they took a jump over, and got away with it, worse luck!" "And you say it was because of treachery on the part of someone on our side?" asked Roger. "That's the story," admitted his chum. "You'll hear the talk as soon as you circulate around a bit. It's a rotten shame, that's what it is!" "But how did anyone from our side get over to the German lines without being shot--unless he took the part of a spy and put on a German uniform?" asked Bob. "They didn't go over--they sent signals," went on Jimmy. "And those signals are what gave away the weak spot in our lines." "Signals!" exclaimed his chums. "What kind?" "Different kinds," replied the young sergeant. "Last night there were light signals, but, of course, signals could be sent by day also, using smoke balls. You know we have a new machine for that." "I didn't know it," admitted Roger. "What is it?" "Well, I saw one up at the signal corps headquarters the other day. It looked like a big soup kettle with a stove pipe sticking out the top, and there were levers on the sides. I asked one of the fellows how it worked, and he showed me. "Of course it's easy enough to make different colored fire signals at night," went on Jimmy. "You've all seen them, even on Fourth of July. But it isn't so easy to signal by smoke in the daytime--or, rather, it wasn't until this machine was invented. Before that they could send up puffs of white or dark smoke, just as the Indians used to signal from one mountain top to the other. "But one of the signal corps men invented this 'smoke kettle,' as I'll call it. The smoke clouds can be made of almost any color--red, green or yellow. And there are white ones, too. It's all done by chemicals. When you pull one lever it sends a certain mixture of chemicals into the caldron. They form a ball of dense, colored smoke. A puff of compressed air sends the ball out up through the 'stove pipe,' as I'll call it, and it sails up into the air, keeping its round shape, like a cloud. "I suppose they have some code, or combination, by which a certain number of smoke balls of a stated color sent up at definite intervals mean something to the man who sees 'em." "Did the traitors send up signals that way?" asked Bob. "I'm not certain of that. All I know is that the smoke balls by day and the colored fires by night are the means used by the regular signal corps. Whether the traitors took a leaf from their book, or stole one of the caldron machines and used it, I don't know. But it gave our weakness away all right, and we got in Dutch." "Rotten work!" exclaimed Roger, and the others agreed with him.