revenge had been taken.

"By all rights you are entitled to a holiday before you join your
new command, under the great Pershing," went on the flight commander.
"However, as I need the services of two brave men to do patrol duty,
I appeal to you. There is a machine gun nest, somewhere in the Boche
lines, that has been doing terrible execution. If you could find
the battery, and signal its location, we might destroy it with our
artillery, and so save many brave lives for France," he went on. "I do
not like to ask you--"

"Tell 'em to get out the machines!" interrupted Jack. "We were just
wishing we could do something to make up for the loss of Harry Leroy,
and this may give it to us. You haven't heard anything of him, have
you?" he asked.

The commander shook his head.

"I fear we shall never hear from him," he said. "Though only yesterday
we received back some of the effects of one of our men who was shot down
behind their lines. I can not understand in Leroy's case."

"Well, we'll make 'em pay a price all right!" declared Tom. "And now
what about this machine gun nest?"

The commander gave them such information as he had. It was not unusual,
such work as Tom and Jack were about to undertake. As the officer
had said, they were practically exempt now that they were about to be
transferred. But they had volunteered, as he probably knew they would.

Two speedy Spad machines were run out for the use of Tom and Jack, each
one to have his own, for the work they were to do was dangerous and they
would have need of speed.

They looked over the machine guns to see that they were in shape for
quick work, and as the one on the machine Tom selected had congealed
oil on the mechanism, having lately returned from a high flight, another
weapon was quickly attached. Nothing receives more care and attention
at an aerodrome than the motor of the plane and the mechanism of the
machine gun. The latter are constructed so as to be easily and quickly
mounted and dismounted, and at the close of each day's flight the guns
are carefully inspected and cleaned ready for the morrow.

"Locate the machine gun battery if you can," was the parting request to
Tom and Jack as they prepared to ascend. "Send back word of the location
as nearly as you can to our batteries, and the men there will see to the
rest."

"We will!" cried the Americans.

Locating a machine gun nest is not as easy as picking out a hostile
battery of heavier guns, for