"And so the great question is settled by the failure of this shell to explode," went on the major. "As soon as we saw it, and noted the absence of the rifling groove marks, we knew it must have been a sub-calibre matter. The rest was easy to figure out.
"Some of us thought there might be a big airship, stationed high above the clouds, dropping bombs. Others inclined to the theory of a double shell; that is, after one had been fired from the cannon it would travel, say, half way and then explode a charge which would impel another shell toward Paris. A sort of cannon within a cannon, so to speak. But this is not so. Nor did the theory of a shell with a sort of propeller device, like that of a torpedo, prove to be right. It is much simpler—just sub-calibre work."
"And what is going to be done about it?" asked Tom. "I mean how can the monster cannon be silenced?"
"Ah, that is a matter we are taking up now," was the answer of Major de Trouville. "I fancy we shall have to call on you boys for a solution of that problem."
"On us?" exclaimed Jack.
"Well, I mean on the aircraft service. It will be their task to search out this great German cannon for us, to enable our gunners to destroy
