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 and 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.