trenches, observing the
movements of his troops, or indicating, by dropping bunches of
tinsel for the sun to shine upon or breaking smoke bombs, the
position of his hidden battery, the foe thus menaced sought to drive
them away with anti-aircraft guns. These proved to be ineffective
and it may be said here that throughout the war the swift airplanes
proved themselves more than a match for the best anti-aircraft
artillery that had been devised. They could complete their
reconnaissances or give their signals at a height out of range of
these guns, or at least so great that the chances of their being hit
were but slight. It was amazing the manner in which an airplane
could navigate a stretch of air full of bursting shrapnel and yet
escape serious injury. The mere puncture, even the repeated
puncture, of the wings did no damage. Only lucky shots that might
pierce the fuel tank, hit the engine, touch an aileron or an
important stay or strut, could affect the machine, while in due
course of time a light armour on the bottom of the fusillage or body
of the machine in which the pilot sat, protected the operator to
some degree. Other considerations, however, finally led to the
rejection of armour.

[Illustration: (C) U. & U.

_A Caproni Triplane_ (_Showing Propellers and Fuselage_).]

Accordingly it soon became the custom of the commanders who saw
their works being spied out by an enemy soaring above to send up one
or more aircraft to challenge the invader and drive him away. This
led to the second step in the development in aerial strategy. It was
perfectly evident that a man could not observe critically a position
and draw maps of it, or seek out the hiding place of massed
batteries and indicate them to his own artillerists, and at the same
time protect himself from assaults. Accordingly the flying corps of
every army gradually became differentiated into observation machines
and fighting machines--or _avions de reglage_, _avions de
bombardement_, and _avions de chasse_, as the French call them. In
their order these titles were applied to heavy slow-moving machines
used for taking photographs and directing artillery fire, more
heavily armed machines of greater weight used in raids and bombing
attacks, and the swift fighting machines, quick to rise high, and
swift to manoeuvre which would protect the former from the enemy, or
drive away the enemy's observation machines as the case might be. In
the form which the belligeren