"Yes, we have one of the Boche sh.e.l.ls," said an officer, who spoke English fluently, for which Tom and Jack were glad. They could speak and understand French, but in a case like this, where they wanted a detailed and scientific explanation, their own tongue would better serve them.

"And can you tell from what sort of gun it comes?" asked Tom.

"It was fired from a monster cannon," was the answer. "That is a cannon not so much a monster in bore, as in length and in its power to impel a missile nearly eighty miles."

"Can it be done?" asked Jack.

"It has been done!" exclaimed Major de Trouville, the officer who was detailed to talk to 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 sh.e.l.l."

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 sh.e.l.l--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 sh.e.l.l. 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 sh.e.l.l 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 sh.e.l.l within a sh.e.l.l,"

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 sh.e.l.l leaves the big gun. If you will imagine a sort of bomb sh.e.l.l 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 sh.e.l.l, 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 sh.e.l.l had chambers for the time fuses to work, much as in a shrapnel sh.e.l.l, 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 sh.e.l.l 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 sh.e.l.l; 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 sh.e.l.l toward Paris. A sort of cannon within a cannon, so to speak. But this is not so. Nor did the theory of a sh.e.l.l 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, 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 sh.e.l.l.

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 sh.e.l.l, 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 sh.e.l.l 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.