"Discovered it!" cried Jack. "Why we didn't see anything!"

"Nothing but those queer lights," added Tom.

Major de Trouville smiled at them, and Lieutenant Laigney nodded his head in a.s.sent.

"Those queer lights, as you call them," said the ordnance expert, "were the flashes of a new explosive. What the Germans call it I do not know.

For want of a better name we call it Barlite, from the name of Professor Barcello, one of our experimenters, who discovered it. But a spy stole the secret and gave it to Germany. They must have managed to perfect it, though we have not used it as yet, owing to the difficulty in constructing a gun strong enough to withstand its terrific power."

"And do you mean they're using this explosive in the big German gun?"

asked Jack, "And that we really saw it being fired?" cried Tom.

"That is my belief," said the lieutenant. "This explosive burns, when fired from a gun, first with an orange flame, changing to green and then to purple, as the various gases are given off."

"Those are the very colors we saw!" exclaimed Jack.

"Yes," went on Major de Trouville. "And when I heard you mention them, and when I recalled that Lieutenant Laigney had spoken of a certain explosive that gave off a tri-colored light, I suspected you had hit on the German secret."

"And do you believe we actually saw the giant cannon being fired at Paris?" asked Tom.

"Without a doubt. The time of the arrival of one of the sh.e.l.ls coincides almost to the minute with the time that would elapse after the missile was sent on its way, and this was when you saw the queer flashes. You have discovered the area where the big gun is placed. All that is needed now are some exact observations to give us the exact spot."

"And then we can destroy it!" cried the lieutenant. "Then the menace to beloved Paris will have pa.s.sed!"

"And thanks to our brave American friends!" cried the major, shaking hands with Tom and Jack. "You will win promotion for this!" he murmured.

"But the big gun isn't found yet," said Jack.

"Why, if you are right, sir," Tom said to the major, "the sh.e.l.ls must pa.s.s right over our camp."

"They probably do. But at so far above--several miles up so as to reach the height of thirty-five--that we never know it. We neither see them nor hear them. Boys, I believe you have located the big gun! All that now remains is to destroy it!"

CHAPTER XXI

DEVASTATING FIRE

Modestly enough Tom and Jack took the new honors that came to them. As a matter of fact they were in no wise sure that they had discovered the location of the German giant cannon. It was all well enough to come in and report seeing some strange-colored flares of fire. But Tom and Jack felt that they wanted to see a thing with their own eyes before surely believing.

Of course, though, the French experts knew about what they were talking, and the major and the lieutenant seemed very sure of their ground.

"I only hope we have had the good luck to have spotted the beasts'

machine," said Tom.

"You will have the honor of proving it to yourselves in the morning,"

Major de Trouville told them. "You shall accompany the first scouting party that goes out. We will send out two photographing machines, and enough of a squadron to meet anything the Huns can put forth. Paris shall be delivered from the Boche pests!"

"We'll do our best," said Tom, and Jack nodded in agreement.

It did not take long for the news to spread about Camp Lincoln that the two young United States aviators had, very probably, discovered by accident the big German gun.

And in telling what they had seen Tom and Jack remarked that the peculiar tri-colored fire had been in the midst of other flashes of flame, and, doubtless, smoke, but that could not be seen on account of the darkness.

"The other flashes were probably guns fired to camouflage the flash from the giant cannon, or possibly cannons," observed Major de Trouville.

"But we shall see what to-morrow brings forth."

The hours of the night seemed long, but there was much to do to get ready for the next day's operations. More aviators were sent for, and the men of the air spent many hours tuning up their motors and seeing to their guns, while the big machines, which it was hoped could take pictures of the giant cannon's position, were gone over carefully.

In addition some powerful French guns were brought up--some of the longest range guns available, and it was hoped that the big aeroplanes might signal by wireless the exact location of the super-gun, so that a devastating fire could be poured on it, as well as bombs be dropped from some machines especially fitted for that work.

Camp Lincoln, where the picked squadron was situated, was in the neighborhood of Soissons, France, in a sector held by the French troops.

The lines of German and French trenches, with No Man's Land in between, was about ten miles to the east of this point. This section had changed hands twice, once being occupied by the Germans, and then abandoned by them when they made the great withdrawal.

Now, perhaps ten miles back of the German trenches, the great gun was hidden, making its total distance from Paris about eighty miles, but its distance from Camp Lincoln something less than twenty miles.

Modern guns easily shoot that distance, but the commander of the forces in this section was going to shorten that. Soissons was the nearest large city to the camp. As a matter of fact the air squadron was some distance east of that place, and nearer the battleline. So that it was comparatively easy, once the location of the big gun was known, to bring up heavy artillery behind the French lines to batter away at its emplacement.

After a night of arduous labor, during which there was anxiety lest the Germans find out what was going on, morning broke, and to the relief of all it was bright.

There was an early breakfast, and then the aviators' helpers wheeled the machines from the hangars. Several big photographing craft were in readiness, and ten bombing planes were in reserve.

Major de Trouville inspected his brave men. They were as eager as dogs on the leash to be off and at the throat of the Huns. A wireless message from Paris had come in soon after breakfast, stating that nearly a score had been killed in the capital the previous night by fire from the "Bertha."

"And it's up to us to avenge them!" exclaimed Jack.

"That is what we'll do if we have any luck!" added Tom grimly.

There was a last consultation of the officers, instructions were gone over, and everything possible done to insure success. The moment a big gun was sighted, the signal was to be given and the French long-range cannon would open fire, while the bombing machines would also do their part.

"All ready! Go!" called the major, and there was a rattle and a roar that drowned his last word. The men of the air were off.

Led by Tom and Jack, the others followed. Up and up they arose, the smaller planes flying high as a protection to the more c.u.mbersome machines of the bi-motored type. And soon the squadron, the largest that had yet ascended from Camp Lincoln, was hovering over the German lines.

The Huns seemed to realize that something more than an ordinary attack from the air was impending, for soon after the anti-aircraft guns began firing a swarm of German aviators took the air, and there was no shirking battle this time. The Huns so evidently felt the desperate need of driving away their attackers, that this, more than what the major and lieutenant had said, convinced Tom and Jack that they were at last on the track of the big gun.

Of course the two boys could not communicate with one another, but they said afterward that their thoughts were the same.

The battle of the air opened with a rush and a roar. The Germans, though outnumbered by their opponents, did not hesitate, but came on fiercely.

They attacked first the big photographing planes, for they realized that these were the real "eyes" of the squadron. The impressions they received, and the views they carried back, might mean the failure of the German plans.

But the French were ready for this, and the swift little Nieuports, dashing here and there, swooping and rising, attacked the other planes vigorously.

It was give and take, hammer and tongs, fire and be fired on, smash and be smashed. It was not as one-sided a battle as it would seem it might have been owing to the superiority of numbers in favor of the French--at least at first. Several of the Allies' planes were sent down, either out of control, or in flames. But the Huns paid dearly for their quarry.

Jack and Tom ran serious risks, for the Germans, realizing that the two leading planes had some special mission, attacked them fiercely. Tom managed to shake off and disable his antagonist. But Jack's man shot with such good aim that he pierced his gasolene tank, and had it not been that Jack was able to thrust into the hole one of some wooden plugs he had brought along for the purpose, he might have had to come down within the German lines. But the wood swelled, filled the hole, and then the petrol came out so slowly that there was comparatively little danger.

And having, with some of their companions, fought their way through the German air patrol, and having escaped with minor damage to their guns, Jack and Tom looked down at the place where they had seen the queer lights.