, 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
it. Or it may be that it will have to be bombed from an aeroplane."

"That's the task I'd like all right!" cried Tom, with shining eyes.

"Same here!" echoed Jack. "Do you suppose we'll get a chance?" he asked
eagerly.

"You may," was the reply. "It may take all the resources of our airmen
to destroy this terror of the Germans. But it will be done, never fear!"

"_Vive la France!_" cried his companions, and there was a cheer in which
Tom and Jack joined.

And so a part of the secret was discovered. It was a monster cannon that
was devastating Paris. A great gun, the construction of which could only
be guessed at. But it must be destroyed! That was certain!

CHAPTER XII

FOR PERILOUS SERVICE

Tom and Jack spent some little time looking at the strange German shell.
It was of peculiar construction, arranged so that the two explosive
charges would detonate together or separately, according as the
mechanism was set.

But in this case it had failed to work, and the shell, falling in a bed
of soft sand, near some new buildings which were going up, had not been
fired by concussion, as might have happened.

"And it was just French luck that it didn't go off," observed Jack.

"That's right," agreed Tom. "If they hadn't had this whole shell to
examine they wouldn't know about the big gun."

So all the theories, fantastic enough some of them, about great airships
hovering over the beautiful city, and dropping bombs from a great
height, were practically disproved.

"Well, now that you have decided it is a big German gun, the next
question is, where is it and what are you going to do about it?"
observed Tom, for he and Jack had been made so much of by the French
officers that they felt quite at home, so to speak.

"Ah, messieurs, that _is_ the question," declared Major de Trouville.
"First to find the gun, and then to destroy it. The first we can do with
some degree of accuracy."

"How?" asked Tom.

The major went to a large map hanging on the wall of the room. It showed
the country around Paris and the various lines as they had been moved to
and fro along the Western front, according as the Germans advanced or
retreated.

"You will observe," said the major, "that by describing an arc, with
Paris as the center of the circle, and a radius of about seventy-five
miles, you