in way, which was the signal for going down. "We've got to get
lower if we want to see anything," the young aviator went on. "Though
they may pot some of us."

Down they went, flying comparatively low but at great speed in order to
offer less of a target to the gunners below them. And, following
instructions, each pilot noted carefully the section of the German
trenches beneath him, and the area back of them. They were seeking the
big gun.

But, though they looked carefully, it could not be seen, and finally
when one of the French machines was badly hit, and the pilot wounded, so
that he had to turn back toward his own lines, Cerfe gave the signal for
the return.

In all this time not a Hun plane had come out to give battle. What the
reason for this was could only be guessed at. It may have been that none
of the German machines was available, or that skillful pilots, capable
of sustaining a fight with the veterans of the French, were not on hand
just then. However that may have been, Tom, Jack and the others, after
firing a few rounds from their machine guns at the trenches, though
without hope of doing much damage, turned back toward Camp Lincoln.

"Well, then you did not discover anything?" asked Major de Trouville,
who had been transferred and given the command at Camp Lincoln.

"Nothing," answered Jack.

"If it's in the section we covered, it is well hidden," added Tom.

"And I think, don't you know," went on the Englishman, Haught, "that the
only way we'll be able to hit on the bally mortar is to fly low and take
photographs."

"That's my idea," said the major. "If we take a series of photographs
we can develop them, enlarge them, if necessary, and examine them at our
leisure. I had thought of this, but it's a slow plan, and it
means--casualties. But I suppose that can't be avoided. But I wanted to
try the scouting machines first.

"After all, the taking of photographs from the air of the enemy trenches
and the land behind them is a most valuable method of getting
information," he continued.

Men, specially trained for such observation work, examine the
photographs after the aviators return with the films, and they can tell,
by signs that an ordinary person would pass over, whether there is a new
battery camouflaged in the vicinity, whether preparations are under way
for receiving a large number of troops, or whether a general advance is
contemplated. Then measures to oppose this can be started. So, Maj