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.