Soaring over Paris, having hastened to take the air when the signal was given, were a number of planes, their red, white and blue lights showing dimly against the black sky. They were off to try to place the big gun, if such it was, or discover whether or not some Hun plane was hovering over the city, dropping the bombs.

As Jack and Tom hastened on, in the wake of the crowd, which was hurrying toward the place where the latest sh.e.l.ls had fallen, again came a distant explosion, showing that the gun had been fired again.

"Fifteen-minute interval," announced Tom, looking at his watch. "They're keeping strictly to schedule."

"Night firing is new for the big gun," said Jack. "I do hope they'll be able to locate the cannon by the flashes."

"It isn't going to be easy," a.s.serted Tom.

"Why not?"

"Because you can make up your mind if the Germans were afraid to fire the piece at night at first for fear of being discovered, and if now they are firing after dark, they have some means of camouflaging the flash. In other words they have it hidden in some way."

"Well, I suppose you're right. But say, Tom, old man! what wouldn't I give to be able to be up in the air with those boys now?" and Jack motioned to the scouts who were flitting around in the dark clouds, seeking for that which menaced the chief city of the French nation.

"I'd like to be there myself," said Tom. "And if this keeps up much longer I'm going to ask permission for us to go up and see what we can do."

"Think they'll let us?"

"Well, they can't any more than turn us down. And we've got to get at it in a hurry, too, or we'll have to report back at our regular station. We aren't doing anything here, except sit around."

"No, we must get busy, that's a fact," said Jack. "It's about time we downed some Hun scout, or broke up one of their 'circus' attacks. I've almost forgotten how a joy stick feels."

A "joy stick" is a contrivance on an aeroplane by the manipulation of which the plane is held on a level keel. If the joy stick control is released, either by accident (say when the pilot is wounded in a fight), or purposely, the plane at once begins to climb, caking its pa.s.senger out of danger.

Once the joy stick is released it gradually comes back toward the pilot. The machine climbs until the angle formed is too great for it to continue, or for the motor to pull it. Then it may stop for an instant when the motor, being heavier, pulls the plane over and there begins the terrible "nose spinning dive," from which there is no escape unless the pilot gets control of his machine again, or manages to reach the joy stick.

"Well, we'll have to get in the game again soon," said Tom. "But what do you say to taking a taxi? This explosion is farther than I thought."

Jack agreed, and they were soon at the place where the last German sh.e.l.l had fallen--that is as near as the police would permit.

A house had been struck, and several persons, two of them children, killed. But, as before, the military damage done was nothing. The Germans might be spreading their gospel of fear, but they were not advancing their army that way.

As Tom and Jack stood near the place where a hole had been blown through the house, another explosion, farther off, was heard, and there was a momentary flare in the sky that told of the arrival of another sh.e.l.l.

For a few seconds there was something like a panic, and then a voice struck up the "Ma.r.s.eillaise," and the crowd joined in. It was their defiance to the savage Hun.

A few shots were fired by the Germans, but none of them did much damage, and then, as though operating on a schedule which must not, under any circ.u.mstances, be changed, the firing ceased, and the crowds once more filled the streets, for it was yet early in the night.

The next morning the boys went to report, as they did each day, expecting that they might be called back to duty. They also found, after being told that their leave was still in effect, that some of the aviators who had gone up the night before, to try to locate the German gun, were on hand.

"Now we can ask them what they saw," suggested Jack.

"That's what we will," a.s.sented Tom.

But the airmen had nothing to report. They had ascended high in search of a hostile craft carrying a big gun, but had seen none.

They had journeyed far over the German lines, hoping to discover the emplacement of the gun, if a long range cannon was being used. But they saw nothing.

"Not even flashes of fire?" asked Tom.

"Oh, yes, we saw those," an aviator said. "But there were so many of them, and in so many and such widely scattered places, that we could not tell which one to bomb. We did manage to hit some, though with what effect we could not tell."

"Then the German gun is still a mystery," observed Tom.

"It is. But we shall discover it soon. We will never rest until we do!"

So more and new and different theories continued to be put forth regarding the big cannon, if such it was. Ordnance experts wrote articles, alike in London, Paris, and New York, explaining that it was possible for a cannon to be within the German lines and still send a sh.e.l.l into the French capital. But few believed that it was feasible.

The general opinion was that the gun was of comparative short range, and was hidden much nearer Paris than the sixty or seventy-odd miles away, beyond which stretched the German line of trenches.

Meanwhile Tom, though making careful inquiries, had learned nothing of his father. He did not feel it would be wise to cable back home, and ask what the news was there.

"It might spoil dad's plans if I did that," said Tom to his chum, "and it would worry the folks in Bridgeton to know that I haven't yet seen him in France. No, I'll just have to wait."

And wait Tom did, though there is no harder task in all the world.

It was one morning, after a night bombardment on the part of the Germans, that Jack, who had been out for a morning paper, came rushing into the room where Tom was just awakening.

"Great news, old man! Great!" cried Jack, waving the paper about his head.

"You mean about a victory?" asked Tom.

"No, not exactly, though it may lead to that. And it isn't any news about your father, I'm sorry to say. It's about the German gun. A 'dud'

fell last night."

"A 'dud'?" repeated Tom, hardly sensing what Jack said.

"Yes, you know! A sh.e.l.l that didn't explode. Now they have a whole one to examine, and they can find out what sort of gun shot it. This paper tells all about it. Come on! Let's go for a look at the 'dud'!"

CHAPTER XI

A MONSTER CANNON

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 sh.e.l.ls 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 sh.e.l.l ought to give 'em a line on the kind of gun that fired it."

"The only trouble," said Jack, "is that the sh.e.l.l 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 sh.e.l.l 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.