Its possible size and the nature of its shooting was discussed every day by Tom, Jack and their comrades. In order to make a cannon shoot a distance of about eighty miles it was known that it was necessary to get the maximum elevation of forty-five degrees. It was also calculated that the sh.e.l.l must describe a trajectory the highest point in the curve of which must be thirty-five miles or more above the earth. In other words the German cannon had to shoot in a curve thirty-five miles upward to have the missile fly to Paris. Of course at that height there was very little air resistance, which probably accounted for the ability of the missile to go so far. That, and the sub-calibre sh.e.l.l, made the seemingly impossible come within the range of possibility.

"What are you going to do, Tom?" asked Jack one evening, after an unsuccessful day's flight. For Tom was going toward his hangar.

"Going up."

"What for?" Jack went on.

"Oh, no reason in particular. I just feel like flying. We didn't do much to-day. Had to come back on account of mist, and we didn't see enough to pay for the petrol used. Want to come along?"

"Oh, I might, yes."

Tom and Jack went up, as did several more. But the two remained up longer than did the others, and Jack was somewhat surprised to see his chum suddenly head for the German lines, but at an angle that would take him over them well to the south of where the observation work had been carried on.

"I wonder what he's up to," mused Jack; "Guess I'd better follow and see."

There was not much chance of an aerial battle at that hour, for dusk was coming on. There had been no bombing squadron sent out, which would have accounted for Tom going to meet them, and Jack wondered greatly at his chum's action.

Still there was no way of asking questions just then, and Jack followed his friend. They sailed over the German lines at a good height, and Jack could keep Tom in view by noting the lights on his plane.

These were also seen by the Germans below, and the anti-aircraft guns began their concert, but without noticeable effect. None of the Hun airmen seemed disposed to accept a challenge to fight, so Tom and Jack had the upper air to themselves.

Below them the boys could see flashes of fire as the various guns were discharged; and at one point in the lines there was quite an artillery duel, the French batteries sending over a shower of high explosive sh.e.l.ls in answer to the challenge from the Boches.

It was not until Jack had followed his chum back to Camp Lincoln, and they had made a landing, that a conversation ensued which was destined to have momentous effect.

"Jack, did you notice the peculiar colored lights away to the north of where we were flying?" asked Tom, as they divested themselves of their fur garments.

"You mean the orange colored flare, that turned to green and then to purple?" asked Jack.

"That's it. I thought you'd see it. I wonder what it means?"

"Oh, perhaps some signal for a barrage or an attack. Or they may have been signaling another battery to try to pot us."

"No, I hardly think so. They didn't look like signal fires. I must ask Major de Trouville about that."

"What?" inquired the major himself, who was pa.s.sing and who heard what Tom said.

"Why, we noticed some peculiar lights as we were flying over the German lines in the dark. There was an orange flare, followed by a green light that changed to purple," answered Tom.

"There was!" cried the major, seemingly much excited. "You don't mean it! That's just what we've been hoping to see! Come, you must tell Laigney about this."

CHAPTER XX

THE BIG GUN

For a moment Tom and Jack did not quite know what to make of the excitement of Major de Trouville. And excited he certainly was beyond a doubt.

"You must come and tell this to Lieutenant Laigney at once," he said.

"It may mean something important. Are you sure of the sequence of the colors?" he asked. "That makes all the difference."

"There was first an orange tint," said Tom, "which was followed by green and purple, the last gradually dying out."

"Orange, green and purple," murmured the major. "Can it be that for which we are seeking?"

He hurried along with the boys, seemingly forgetting, in his haste and excitement, that he was their ranking officer. But, as has been noted, the aviators are more like friends and equals than officers and men.

There is discipline, of course, but there is none of the rigidity seen in other branches of the army. In fact the very nature of the work makes for comradeship.

Tom and Jack knew, slightly, the officer to whom Major de Trouville referred. Lieutenant Laigney was an ordnance expert, and the inventor of a certain explosive just beginning to be used in the French sh.e.l.ls. It was simple, but very powerful.

"You must tell him what you observed--the strange colored lights, my boys," said the major. "By the way, I hope you carefully noted the time of the colored flares."

Tom and Jack had. That was part of their training, to keep a note of extraordinary happenings and the time. Often seemingly slight matters have an important bearing on the future.

They found Lieutenant Laigney in his quarters, making what seemed to be some intricate calculations. He saluted the major and nodded to the boys, whom he had met before.

"Lieutenant," began Major de Trouville, "these young gentlemen have something to tell you. I want you to think it over in the light of what you told me about the action of that new explosive you said the Germans might possibly be using."

"Very good, Major. I shall be delighted to be of any service in my power," was the answer.

Then Tom and Jack described what they had seen, giving the location of the colored lights as nearly as they could, and the exact time they had noted them.

"How long would it take a sh.e.l.l to reach Paris, fired at a distance of eighty miles from the city?" asked the major.

The lieutenant made some calculations, and announced the result of his findings.

"Then," went on the commanding officer, "if a sh.e.l.l was fired from the big gun, say at the moment when these two scouts observed the tri-colored fire, it should have reached Paris at seven-fifty-three o'clock."

"As nearly as can be calculated, not knowing the exact speed of the projectile, yes," answered the lieutenant.

Major de Trouville picked up the telephone and asked to be connected with the wireless station.

"Have you had any reports of the bombarding of Paris this evening?" he asked. "Yes? What time did the first, or any particular sh.e.l.l, arrive?

Ah, yes, thank you. That is all at present."

He turned to the others, after having listened to the reply and put the instrument away.

"One of the sh.e.l.ls exploded in a Paris street at seven-fifty-two o'clock this evening," he said.

"It beat your calculations by one minute, Lieutenant Laigney."

"Ah! Then this means--" and the younger officer seemed as excited as the major had been when Tom and Jack told him what they had seen.

"It means," finished the commanding officer, "that, in all likelihood, these young men have discovered the location of the big German cannon."