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Tom, dressing hastily, read the account in the Paris paper of the fall, in an outlying section of the city, of one of the German shells that failed to explode. It was being examined by the military authorities, it was stated, with a view to finding out what sort of gun fired it, so that measures might be taken to blow up the piece or render it useless to the enemy.

"That sounds good to me," said Tom, as they made a hasty breakfast. "This is getting down to a scientific basis. An unexploded shell ought to give 'em a line on the kind of gun that fired it."

"The only trouble," said Jack, "is that the shell may go off when they are examining it."

"Oh, trust the French ordnance experts not to let a thing like that happen," said Tom. "Now let's go to it."

It was fortunate that Tom and Jack wore the uniforms that had so endeared them to France, or they might have had difficulty in gaining admittance to the bureau where the unexploded shell was under process of investigation. But when they first applied, their request was referred to a grizzled veteran who smiled kindly at them, patted them on the shoulders, called them the saviors of France, and ushered them into the ordnance department, where special deputies were in conference.

"Yes, we have one of the Boche shells," 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 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 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.

"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, 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!