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