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.
Tom's work was done, and he flew off to the aid of Jack, who had overcome one Hun, sending his plane crashing to earth. But the other, an expert fighter, was pressing him hard until Ton opened up on him with his machine gun. Then the German, having no stomach for odds, turned tail and flew toward his own lines.
“Good for you, Tom!” yelled Jack, though he knew his chum could not hear him because of the noise of the motor.
Together the two lads, who had engaged in their last battle strictly with the French, made for their aerodrome, reaching it safely, though, as it was learned when Jack dismounted, he had received a slight bullet wound in one side from a missile sent by one of the attacking planes. But the hurt was only a flesh wound; though, had it gone an inch to one side, it would have ended Jack's fighting days.