On the contrary, it was his withdrawal which was regulated by the speed of our advance. There was not a foot of ground which was not contested by all the effort which the enemy was able to put forth. It is quite true that his withdrawal was intentional; but it is not true that it was conducted at the deliberate rate which was necessary to enable him to withdraw in good order.
He was compelled to fight all the time and to withdraw in disorder. He was forced to abandon guns and huge quantities of stores. The amount of derelict artillery ammunition found scattered over the whole of this considerable area alone reached hundreds of thousands of rounds, distributed in hundreds of dumps and depots, as well as scores of tons of empty artillery cartridge-cases, the brass of which had become of priceless value to the enemy.
Regimental and even Divisional Headquarters were abandoned as they stood, with all their furniture and mess equipment left intact. Signal wire and telephone equipment remained installed in all directions, hospitals and dressing-stations were left to their fate. The advance yielded to us over 600 prisoners, some half-dozen field-guns, and large numbers of smaller weapons.
The last two days of the advance led us across a maze of trenches and the debris of the 1916 campaign. The weather was unfavourable, there was much rain and an entire absence of any kind of shelter. As a result the line Brigades had to put forth all their powers of endurance and reached the Somme in a very tired condition.
In the meantime my air squadron had an exceptionally busy time. Contact patrols were maintained throughout every hour of daylight. Difficult as it was to identify the positions reached by our leading troops during an organized battle, where their approximate positions and ultimate objective lines were known beforehand, it was doubly so when no guide whatever existed as to the probable extent of each day's advance, or as to the amount of resistance likely to be encountered at different parts of the front.
Yet it was just under these circumstances that rapid and reliable information as to the progress of the various elements of our front line troops was more important than ever, and no means for obtaining such information was so expeditious as the Contact Aeroplane.
To assist the air observer in identifying our troops, the latter were provided with flares, of colours which were varied from time to time in order to minimize the risk of imitation by the enemy. The method of their employment, whether singly or in pairs, or three at a time, was also frequently varied.
These flares on being lit gave out a dense cloud of coloured smoke, easily distinguishable from a moderate height. The contact plane, which would carry coloured streamers so that the infantry could identify it as flying on that particular duty, would, when ready to observe, blow its horn and thereupon the foremost infantry would light their flares.
It was a method of inter-communication between air and ground, which, after a little practice, came to be well understood and intelligently carried out. By its means a Divisional or Brigade Commander was kept accurately informed, with great promptitude, of the progress of each of his front line units, in relation to the various woods, ruined mills, and other obstacles which lay spread across their path.
But the Air Force had another interesting duty, which was to watch the roads leading back from the enemy's front line to his rear areas. During tranquil times little movement could ever be seen on the enemy's roads in the hours of daylight, for the very good reason that he took care to carry out all his transportation to and from his front zone under cover of darkness.