ted, was not allowed to lapse; it was
attached to No. 4 Squadron, and went out with it to France. The pilots
of this section, Lieutenants Lewis, James, and Winfield Smith, worked
with the squadron, but spent most of their time in making ready the
wireless telegraphy equipment which, when once the retreat was ended and
ground stations were established on a fixed front, came into effective
use. Again, the very rapid development of an efficient German
anti-aircraft service, and the equally rapid improvement in range and
accuracy of anti-aircraft guns, changed the conditions of
reconnaissance. In the almost pastoral simplicity of the first days of
the war, four thousand feet was held to be a sufficient height for
immunity from the effect of fire from the ground. Before long four times
that height gave no such immunity. Machines, therefore, had to be built
to climb quickly, and had to be given a higher 'ceiling', as it is
called; that is, they had to be able to maintain level flight in a more
rarefied medium. But observation with the human eye from a height of
several miles is almost useless for the detective work of military
reconnaissance. So it came about that the improvement of the enemy's
anti-aircraft artillery gave a direct impulse to the improvement of our
aerial photography. A photograph, taken in a good light and enlarged,
reveals many things invisible to the naked eye; a series of photographs
reveals those changes in the appearance of the earth's surface which
result from the digging of new trenches or gun-positions and the making
of new ammunition-dumps.

Improvements in mechanical science, to be of any use in war, depend on
the skill and practice of those who use them. General Trenchard never
forgot this. He thought first of the pilot, and then of the gadgets.
'The good gun-mounting', he once said, 'is the mounting that the pilot
can work.' This was a thing essential to remember at a time when the
pilot got the best part of his training in the war itself. If he could
not work the gun-mounting, the gun-mounting would probably survive him.
To study the tastes and preferences of pilots, even when these tastes
were prejudices, was the only way to efficiency. At the beginning of
1916 General Trenchard made it a rule to supply one experimental
machine, without standardized mountings, to every squadron of the Royal
Flying Corps, so that the pilots might put their own ideas to the test
of practice. They had had but litt