The main objects of contact patrols were to assist the telephone (which was frequently cut by shellfire), to keep the various headquarters informed of the progress of their troops during the attack, so also saving them from the possibility of coming under the fire of their own artillery, to report on enemy positions, to transmit messages from the troops engaged to the headquarters of their units, to attack ground formations, and to co-operate with tanks. A system of red flares on the floor of the trenches was used to mark the disposition of the troops, and aircraft communicated their information by means of signalling lamps, wireless and message-bags. During the German retreat of 1917 contact patrols attacked enemy foundations from 100 feet and in some cases landed behind the enemy lines to obtain information. The skill of low-flying pilots in taking cover by flying behind woods, houses, etc., became increasingly important. The fact that 62,673 rounds of ammunition were fired from the air against enemy ground targets between November 20th and 26th, 1917, and 163,567 between March 13th and 18th, 1918, indicates the rapid development of this form of aircraft action, the effect of which was frequently more deadly than bombing. Two of many protagonists of contact patrol were Pretyman and Bishop. On one occasion the latter, in attacking an aerodrome at about 50 feet, riddled the officers' and men's quarters with bullets, put two or three machines on the ground out of action, and three in succession as they got into the air. Another interesting example of contact patrol work occurred in 1917 when a pilot flew his machine at a low altitude over the enemy trenches, and he and his observer attracted the attention of the Germans by firing their machine guns and Verey lights. The Germans were so busy with the aeroplane that they had their backs turned to the front line and our infantry were able to cross no-man's land without any artillery preparation, take prisoners and bomb dug-outs. An article in the _Cologne Gazette_ showed what the Germans thought of low "strafing." "The operations" (i.e. of June 7th, 1917), it says, "were prefaced by innumerable enemy airmen, who, at the beginning of the preparation for the attack, appeared like a swarm of locusts and swamped the front. They also work on cunningly calculated methods. Their habit is to work in three layers--one quite high, one in the middle, and the third quite low. The English who fly lowest show an immense insolence; they came down to 200 metres and shot at our troops with their machine guns, which are specially adapted to this purpose."