he distance from a great height sufficiently well
to make a direct hit.

But a certain percentage of the bombs find their mark, and they did in
this case. Tom and Jack, as well as the other scouts, looking down from
their planes, saw fountains of brown earth being tossed into the air as
the French bombs exploded. At the same time the photographers in the
other planes were making pictures of the guns and their location.

They were hindered in this not only by the shooting of the Germans from
below, who were working their anti-aircraft guns to their capacity, but
by screens of smoke clouds, which were emitted by a special apparatus to
hide the big guns. At the same time other cannons were being fired to
disguise the sound from the immense long-range weapon, but this was of
little effect, now that the location had been discovered.

Meanwhile a score or more of the Hun planes appeared in the air. They
had taken flight as soon as their pilots saw the squadron of enemy
machines approaching, and were eager, this time, to give battle.

"Our work's being cut out for us," murmured Tom, as he steered his
machine to engage a German who seemed eager for the fray. Tom sent a
spray of bullets at his enemy, and was fired at in turn. He knew his
craft had been hit several times, but he did not think it was seriously
damaged.

Jack, too, as he could tell by a quick glance, was also engaged with a
German, but Tom had no time then to bestow on mere observation. His
antagonist was a desperate Hun, bent on the utter destruction of Tom's
machine. They came to closer quarters.

Down below the fighting was growing more furious. It was in the form of
an artillery duel. For now the French observation machines were
wirelessing back the range, and French shells were falling very near the
big guns.

The heavy guns, in modern warfare, are placed miles away from the
objects they wish to hit, and the only way to know where the targets are
is by aeroplane observation. When the guns are ready to fire one of the
artillery control planes goes up over the enemy's territory. Of course
it is the object of the enemy to drive it away if possible.

But, hovering in the air, the observer in the double-motored machine
notes the effect of the first shot from his side's cannon. If it goes
beyond the mark he so signals by wireless. If it falls short he sends
another signal. Thus the range is corrected, and finally he sees that
the big shells are landing just