The raids were repeated at irregular intervals, owing to atmospheric conditions, throughout the winter until Whitsunday 19 May, when 44 were killed and 179 injured. Generally they occurred when the moon was nearly full, but on 6 December there was one when it was in its last quarter and on 18 December another when it was only four days old, and on 7 March 20 were killed and 55 injured in a raid on a moonless night. On 19 October these aeroplane raids were varied by a raid on a moonless night by Zeppelins which shut off their engines and drifted across London with a north-west wind, dropping only three bombs but killing 27 and injuring 53 persons. Six of the raiders failed to get home, and this was the last of the Zeppelin so far as London was concerned, though Zeppelin raids were made as late as 12 and 13 March on the north-east coast. Reprisals were adopted as a policy by the British Government in the autumn of 1917, and great store was set upon them in some quarters. But in spite of the vigour with which they were carried out along the Rhine, there is no reason to suppose that our aeroplane raids achieved any greater military effect than that which we had always denied to German raids on England. They certainly did not succeed in curing the Germans of their raiding propensities. That was effected by our improvements in defence, notably in our antiaircraft bullets and "aprons" suspended from balloons; and after Whitsunday, 1918, the Germans concentrated on the French, although they had shown fewer qualms about reprisals. Nor did our supremacy in the air produce the effects which many anticipated on the field of battle. Italian superiority with that arm was of little use at Caporetto, and our superiority did not materially further our advance in Flanders in the autumn of 1917 or retard the German offensive at St. Quentin in the spring of 1918. Aircraft were indispensable as eyes for an army, and to a lesser extent for a navy; but as an independent force they were as limited in their effectiveness as is artillery or cavalry without the fundamental infantry.