sought, and temporary aerodromes improvised
every few days. The small collection of serviceable aeroplanes again
justified themselves at the decisive stand in the Marne and Ourcq basin,
where immediate reports of enemy concentrations were essential to
victory. Again, after the Hun had been swept across the Aisne and was
stretching north-eastward tentacles to clutch as much of the coast as
was consonant with an unbroken line, the aerial spying out of the
succeeding phases of retirement was of great service. Indeed, tentative
though it was, the work of the British, French, and German machines
before the advent of trench warfare proved how greatly air
reconnaissance would alter the whole perspective of an open country
campaign.

After the long barrier of trenches deadlocked the chances of extended
movement and opened the dreary months of more or less stationary
warfare, the R.F.C. organisation in France had time and space for
self-development. Aerodromes were selected and erected, the older and
less satisfactory types of machine were replaced by the stable B.E2.C.,
the active service squadrons were reconstructed and multiplied.

To the observation of what happened behind the actual front was added
the mapping of the enemy's intricate trench-mosaic. For a month or two
this was accomplished by the methodical sketches of a few observers. It
was an exceedingly difficult task to trace every trench and sap and to
pattern the network from a height of about 2000 feet, but the infantry
found small ground for dissatisfaction as regards the accuracy or
completeness of the observers' drawings. Then came the introduction of
aerial photography on a large scale, and with it a complete bird's-eye
plan of all enemy defence works, pieced together from a series of
overhead snapshots that reproduced the complete trench-line, even to
such details as barbed wire. By the infallible revelations of the
camera, untricked by camouflage, concealed gun positions were spotted
for the benefit of our artillery, and highly useful information about
likely objectives was provided for the bombing craft.

The frequent bombing of German supply centres in Belgium and North
France came into being with the development of aerial photography. Owing
to the difficulty of correct aim, before the advent of modern
bomb-sights, all the early raids were carried out from a low altitude,
sometimes from only a few hundred feet. For every purpose, moreover, low
altitudes