me to send word to the French battery that
would result in the destruction of the Hun machine nest.

There came a burst of fire from the Allied lines he had left, and Tom
saw a shell land to the left and far beyond the Hun battery hidden in
the old ruins. He at once sent back a correcting signal.

The more a gun is elevated up to a certain point, the farther it shoots.
Forty-three degrees is about the maximum elevation. Again, if a gun is
elevated too high it shoots over instead of directly at the target aimed
at. It is then necessary to lower the elevation. Tom has seen that the
guns of the French battery, which were seeking to destroy the machine
gun nest were shooting beyond the mark. Accordingly they were told to
depress their muzzles.

This was done, but still the shells fell to the left, and an additional
correction was necessary. It is comparatively easy to make corrections
in elevation or depression that will rectify errors in shooting short
of or beyond a mark. It is not so easy to make the same corrections in
what, for the sake of simplicity, may be called right or left errors,
that is horizontal firing. To make these corrections it becomes needful
to inscribe imaginary circles about the target, in this case the machine
gun nest.

These circles are named from the letters of the alphabet. For instance,
a circle drawn three hundred yards around a Hun battery as a center
might be designated A. The next circle, two hundred yards less in size,
would be B and so on, down to perhaps five yards, and that is getting
very close.

The circles are further divided, as a piece of pie is cut, into twelve
sectors, and numbered from 1 to 12. The last sector is due north, while
6 would be due south, 3 east, and 9 west, with the other figures for
northeast, southwest, and so on.

If a shot falls in the fifty-yard circle, indicated by the letter D,
but to the southwest of the mark, it is necessary to indicate that by
sending the message "D-7," which would mean that, speaking according to
the points of the compass, the missile had fallen within fifty yards of
the mark, but to the south-southwest of it, and correction must be made
accordingly.

Tom watched the falling shells. They came nearer and nearer to the
hidden battery and at last he saw one fall plump where it was needed.
There was a great puff of smoke, and when it had blown away there was
only a hole in the ground where the ruins had been hiding the machine
guns.