It was explained, however, when he read the note.  A glance at the signature told him it was from “Captain Frank Dickerson.”

“Boys, you only did your duty in exposing me, as you thought you did,” wrote the officer.  “I congratulate you on your nerve, and on doing what you so plainly disliked to do, after I had saved your lives, as I may flatter myself I did.

“So don’t worry about me.  I was only doing my duty, too, for Uncle Sam when I was within the German lines and in a German uniform.  And I was also doing my duty when I was within your lines in an American uniform.  My superior officers know all about it.  That is all I can say now, except to add that I was not under arrest very long.  But that action had to be taken to keep my plans from becoming known, even to the major.  I hope to meet you all again.”

“Say, what does it all mean?” asked Jimmy, to whom so many things had happened in the last few hours that it was no wonder he was a bit dazed.  “What’s all this talk about the government knowing he was in German uniform and all that?”

“Don’t you understand?” inquired Bob, with a smile.  “He was a spy.”

“Of course he was a spy!” asserted Jimmy.  “I sized that up all right.  He was a spy inside our lines and—­”

“Yes, but he was also a spy inside the German lines,” put in Roger.  “Don’t you understand, Blazes!  Captain Dickerson wore the German uniform to get possession of some of their secrets.  He’s in the United States Secret Service.”

Jimmy looked first at one and then at the other of his chums, until he had faced them all in turn.

“Gee!” he exclaimed at length.  “What a chump I was not to guess that, when he acted so coolly after I denounced him!  What a chump I was!”

“Oh, well, we couldn’t guess everything,” said Franz, “And he certainly acted suspiciously at times.”

“Yes, so I dinks myself,” agreed Iggy, who had not spoken for some time.

“Well, it’s all over—­at least we’ve cleared up two mysteries,” observed Bob.  “I wonder what will happen next?”

“Well, there’s going to be more fighting; that’s sure,” declared Jimmy, “and I want to do my share!”

“Same here!” echoed his chums.

And whether they did or not will be told in our next volume, entitled, “The Khaki Boys Fighting to Win; or, Smashing the German Lines.”

THE END