the boys "It has been done. That is the gun that has
been bombarding Paris."

"But, from a military standpoint," began Tom, "is it--"

"It is utterly useless," was the quick answer. "Come, I will show you
the shell."

He led them to an apartment set aside for the testing of explosives and
working out ordnance problems, and there on a table, around which sat
many prominent French officials, was the German shell--the "dud," as
Jack had called it.

"The charge has been drawn," explained Major de Trouville, "so there is
no danger. And we have determined that the manner in which shots reach
Paris from a distance of from seventy to eighty miles is by the use of a
sub-calibre missile."

"A sub-calibre?" murmured Tom.

"Yes. You know, in general, that the more powder you use, and the larger
the surface of the missile which receives it, the greater distance it
can be thrown, providing your angle of elevation is proper."

The boys understood this much, in theory at least.

"Well," went on the major, "while that is true, there is a limit to it.
That is to say you could go on using powder up to hundreds of pounds in
your cannon, but when you get to a certain point you have to so increase
the length of the gun, and the size of the breech to make it withstand
the terrific pressure of gases, that it is impracticable to go any
further. So, also, in the case of the shell. If you make it too large,
so as to get a big surface area for the gases of the burning powder to
act upon, you get your shell too heavy to handle.

"Now of course the lighter a missile is, the farther it will go, in
comparison to a heavy one with the same force behind it. But you can not
get lightness and sufficient resistance to pressure without size, and
here is where the sub-calibre comes in."

"In other words the Germans have been firing a shell within a shell,"
broke in another officer.

"Exactly," said Major de Trouville. "The Germans have evolved a big gun,
that is big as regards length, to enable the missile they fire from it
to gain enough impulse from the powder. But the missile would be too
large to travel all the way to Paris. So they use two. The inner one is
the one that really gets here and explodes."

"What becomes of the outer?" asked Jack.

"It is a sort of container, or collar, and falls off soon after the
shell leaves the big gun. If you will imagine a sort of bomb shell being
enclosed in an iron case, the whole being put in a gun a