and the aeroplane became accepted units of warfare it
was only natural that efforts should be concentrated upon the evolution
of ways and means to compass their destruction or, at least, to restrict
their field of activity. But aircraft appeared to have an immense
advantage in combat. They possess virtually unlimited space in which to
manoeuvre, and are able to select the elevation from which to hurl their
missiles of destruction.

There is another and even more important factor in their favour. A
projectile fired, or even dropped, from a height, say of 5,000 feet,
is favourably affected by the force of gravity, with the result that
it travels towards the earth with accumulating energy and strikes the
ground with decisive force.

On the other hand, a missile discharged into space from a weapon on the
earth has to combat this action of gravity, which exercises a powerful
nullifying influence upon its flight and velocity, far in excess of
the mere resistance offered by the air. In other words, whereas the
projectile launched from aloft has the downward pull of the earth or
gravitational force in its favour, the shell fired from the ground in
the reverse direction has to contend against this downward pull and its
decelerating effect.

At the time when aircraft entered the realms of warfare very little
was known concerning the altitudes to which projectiles could be
hurled deliberately. Certain conclusive information upon this point was
available in connection with heavy howitzer fire, based on calculations
of the respective angles at which the projectile rose into the air and
fell to the ground, and of the time the missile took to complete its
flight from the gun to the objective. But howitzer fire against aircraft
was a sheer impossibility: it was like using a six-inch gun to kill a
fly on a window pane at a thousand yards' range. Some years ago
certain experiments in aerial firing with a rifle were undertaken
in Switzerland. The weapon was set vertically muzzle upwards and
discharged. From the time which elapsed between the issue of the bullet
from the muzzle until it struck the earth it was possible to make
certain deductions, from which it was estimated that the bullet reached
an altitude of 600 feet or so. But this was merely conjecture.

Consequently when artillerists entered upon the study of fighting
air-craft with small arms and light guns, they were compelled to
struggle in the dark to a very pronounced exte