traordinary or signal services were rendered by the Flying Corps
during the crises of the battle. The weather was bad, and on some days
flying was impossible. Yet by every flight knowledge was increased. When
the British troops arrived in Flanders and were sent at once into the
battle, the country in front of them was unknown. The dispositions of
the enemy forces were not even guessed at. Then by the aid of the Flying
Corps the enemy's batteries were mapped out, his trench lines observed
and noted, his railheads and his roads watched for signs of movement.
The reports received just before the battle do not, it is true, indicate
the whole volume of movement that was coming towards the Ypres area. The
newly raised reserve corps which formed part of the German Fourth Army,
the transport of which to the western front began on the 10th of
October, were not definitely seen from the air until just before the
battle. But observers' reports did indicate that many troops were
moving on the Ypres front, and once battle was joined enemy movements
were fully reported on.

When at the end of October the Belgian army mortgaged great tracts of
their ground for many years by opening the canal sluices and letting in
the sea, the Germans were enabled to divert the Third Reserve Corps
southwards. The movements of troops from this area were observed by the
Royal Flying Corps, and General Headquarters on the 1st of November
issued this summary: 'The coast road from Ostend to Nieuport was
reported clear this morning, and there are indications generally of a
transference of troops from the north of Dixmude southwards.' Again,
when the attack on Ypres had failed and died away, the Germans
transferred many troops from the western to the eastern front; these
movements also were seen by the Royal Flying Corps, who reported on the
20th of November an abnormal amount of rolling stock at various stations
behind the German front. 'The rolling stock formerly parked on the
Ostend-Thourout and Ostend-Roulers lines has evidently been broken up',
says General Headquarters Intelligence Summary for the 20th of November,
'and distributed to a number of stations along the Lys and in the area
immediately north of it, which would be suitable points of entrainment
for the forces in that district.... This redistribution of the rolling
stock, together with the apparent reduction in motor transport, would
seem to point to some important movement away from this immedi