!" 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 shells. 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 shell to reach Paris, fired at a distance of
eighty miles from t