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
