axicab driver was that a lady and a girl,
answering the descriptions of Bessie and her mother, had been met in
front of the restaurant by a man.

The three, after a short talk, had driven off together in an automobile,
and that was the last seen of them.

"But we'll get some trace," declared the detective. "It is hard to get
in or out of Paris now without proper papers. And while, of course, this
spy may have forged documents, there is a chance that we may intercept
him and help your friends. Time is against us, but we will do our best."

Tom and Jack knew that. There was nothing else to do, and so, worried as
they were, they went back to their comrades. Tom made some inquiries
about his father, but, as he feared, no news had come.

As may be imagined Tom and Jack did not pass a very restful night. The
Zeppelin raid had set their nerves on edge, as well as those of every
one else, and it could not be told when the big gun might begin firing
again. Then the fact of Mrs. Gleason and Bessie being missing, and not
knowing in what danger they might be, added to the boys' anxiety.

They paid a late visit to the police, hoping for news, but the spy had
not been apprehended. Then they hurried back to get a little rest
before starting with their comrades of the air to search for the monster
gun.

While these events were transpiring, the French army intelligence
department had not been idle. The officials knew how vitally necessary
it was, in order not to have the morale of the people of Paris weakened,
to do something to find and silence the big guns. And first it was
necessary to discover them.

While this, as yet, had not been done with exactness, owing to the
concealing tactics of the Germans, it was believed that the long-range
cannon was hidden in a certain wood near Laon. French airmen had
endeavored to spy out certain positions there, but an unusually large
number of German planes had fought them off.

"That's pretty good evidence that there must be something doing,"
observed Tom, when he heard this information. "Laon is about ten miles
behind the German lines as they exist at present. Just a breather for a
good French plane. Jack, that's a trip we'll soon be taking."

"I'll be with you, old scout. How's your hand?"

"Oh, all right now. I can hold the joy stick or work the gun. I'm ready
for whatever comes along."

The time had come for the picked squadron to leave Paris and assemble
at the aerodrome assigned