e four years of War than it had done during the six or seven years
preceding it.

It is impossible in the space at disposal to treat of this development
even with the meagre amount of detail that has been possible while
covering the 'settling down' period from 1911 to 1914, and it is
proposed, therefore, to indicate the improvements by sketching briefly
the more noticeable difference in various respects between the average
machine of 1914 and a similar machine of 1918.

In the first place, it was soon found that it was possible to obtain
greater efficiency and, in particular, higher speeds, from tractor
machines than from pusher machines with the air screw behind the main
planes. This was for a variety of reasons connected with the efficiency
of propellers and the possibility of reducing resistance to a greater
extent in tractor machines by using a 'stream-line' fuselage (or body)
to connect the main planes with the tail. Full advantage of this could
not be taken, however, owing to the difficulty of fixing a machine-gun
in a forward direction owing to the presence of the propeller. This was
finally overcome by an ingenious device (known as an 'Interrupter gear')
which allowed the gun to fire only when none of the propeller blades
was passing in front of the muzzle. The monoplane gradually fell into
desuetude, mainly owing to the difficulty of making that type adequately
strong without it becoming prohibitively heavy, and also because of its
high landing speed and general lack of manoeuvrability. The triplane
was also little used except in one or two instances, and, practically
speaking, every machine was of the biplane tractor type.

A careful consideration of the salient features leading to maximum
efficiency in aeroplanes--particularly in regard to speed and climb,
which were the two most important military requirements--showed that
a vital feature was the reduction in the amount of weight lifted per
horse-power employed; which in 1914 averaged from 20 to 25 lbs. This was
effected both by gradual increase in the power and size of the engines
used and by great improvement in their detailed design (by increasing
compression ratio and saving weight whenever possible); with the result
that the motive power of single-seater aeroplanes rose from 80 and 100
horse-power in 1914 to an average of 200 to 300 horse-power, while the
actual weight of the engine fell from 3 1/2-4 lbs. per horse-power to an
average of 2 1/2 lbs. pe