d for the purpose, and this practice was continued with success
throughout the Battle of the Aisne.

In the earliest stages artillery co-operation was also carried out by
dropping coloured lights, but from the Battle of Ypres onwards, though
for some time very few wireless machines were available, this was
effected by wireless or signal lamps. In his dispatch on the Battle of
Loos, Sir John French wrote: "The work of observation for the guns from
aeroplanes has now become an important factor in artillery fire, and the
personnel of the two arms work in closest co-operation."

By the Battle of the Somme artillery co-operation had assumed very large
dimensions. For instance, on September 15th, 1916, on the front of the
4th Army alone, seventy hostile batteries were located, twenty-nine
being silenced. Counter-battery work was so effective before the
offensive which opened on the Ypres front at the end of July, 1917, that
the Germans withdrew their guns and the attack was delayed for three
days in order that their new positions might be located.

Recognition marks on aeroplanes were at this time, and indeed throughout
the war, a matter of great difficulty. It had been suggested before the
war that they would not be necessary, but the reverse was found to be
the case, as even with the distinctive marks which were adopted our
machines were often fired at by British troops, and we should
undoubtedly have lost very heavily if we had flown over our own lines
with false marks, as was suggested, or none.

_Bombing._

The bombing operations, which reached their climax in the raids on
German industrial centres in 1918, arose from very primitive methods
used at the beginning of the war. During the retreat from Mons a few
hand grenades were carried experimentally in the pockets of pilots and
observers, or, in the case of the larger varieties, tied to their
bodies, and these were dropped over the side of the machine as
opportunity occurred. At the Marne, for instance, small petrol bombs set
fire to a transport park and scattered a mixed column of infantry and
transport. I think I am right in saying that the first German bombs
were dropped on us--unsuccessfully--at Compiegne on August 29th, 1914.
It was not, however, until the beginning of 1915 that special bombing
raids were started by the Royal Flying Corps, one of the first places to
be attacked being the Ghistelles aerodrome in West Flanders.

The most important bombing oper