nd fired, you
will better get the idea. The outer case is made in two or more pieces,
and soon after it is shot out it falls away, leaving the smaller missile
to travel on. But here is where the cunning of the invention comes in.
The smaller missile has all the impetus given the larger one, but
without its weight. In consequence it can travel through eighty miles of
atmosphere, finally reaching Paris, where it explodes."

"Wonderful!" exclaimed Jack.

"And yet it is merely the adaptation of an old theory," went on the
major. "We have known of the sub-calibre theory for years, but it is not
practicable. So we did not try it. The cost is too great for the amount
of military damage done. And this shell, as you will see, is composed of
two parts, each with a separate explosive chamber, each containing, as
we discovered, a different sort of explosive. In this way if one did not
go off, the other would, and so set off the one that failed. It is very
clever, but we shall be more clever."

"That's right!" chimed in a chorus of fellow officers.

"We'll find the gun and destroy it--or all of them if they have more
than one, as they probably have," went on the major.

He showed the boys where the shell had chambers for the time fuses to
work, much as in a shrapnel shell, which can be set to go off so many
minutes or so many seconds after it reaches its objective point.

"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