t chance you get, and let out a whoop at
the same time. It'll give 'em a shock, and start 'em to running. Then
we'll soon have a pack on their heels."

"What if they use their trench grenades on us?" asked Tom.

"Not likely to except as a last resort. But we've got to take the
chances. Dodge, that's all. Now to swing around the big hangar."

Harry squeezed his companion's arm while saying this. Both of them were
fairly quivering with the excitement, just as highly strung race horses
can be seen quivering while prancing up and down awaiting the tap of the
bell that summons them to the wire for the start.

It was not pitch-dark, but even the possessor of keen eyesight would
have had to look closely in order to make certain that a moving object
was a human being and not a dog.

Harry's surmise proved to be well founded, for they quickly discovered a
suspicious movement close to the large hangar. Yes, the two Hun spies
were undoubtedly there, and already busily engaged in doing something
that could only mean trouble for the American escadrille.

Closer the pair of watchers crept. They could now hear the men
whispering as they worked, and Tom even believed he caught a guttural
German word used. This convinced him their theory was founded on fact,
and that these were secret enemies in the camp.

Another half minute and he felt Harry nudge him. That meant the other
believed the time had arrived for them to make their leap; and when he
felt his companion start Tom stirred himself.

Both let out a yell as they sprang forward. Tom more than half expected
to hear an explosion, thinking the Huns, on finding themselves caught in
the act, would fire their grenades promptly.

Nothing of the kind came about. Instead both men instantly dropped flat
and started to roll away with incredible swiftness, as though escape was
the first thought in their minds.

Tom hurled himself through space. His intention was to pin one of the
spies to the ground and try to hold him there until help came. Their
outcries would of course arouse every man within hundreds of yards of
the spot, and lights must soon be brought to bear on the scene.

Although Tom's calculations may have been all right, he did not meet
with as much success as he had probably anticipated. Perhaps the wary
Boche guessed what was coming; at any rate he succeeded in squirming
from under, and when Tom landed it was only to feel the other rolling
out of his reach.

But he w