f 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 shells 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 passed!"

"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 shells must
pass 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