On the other hand, aeroplane daylight and night raids on London, the first of which occurred in November, 1916, increased in number and strength with the object, in addition to the destruction of material and civilian _morale_, of forcing upon us the unsound retention at home of a considerable air defence force. The largest of these attacks was made by seventeen aeroplanes at midday on June 13th, 1917, but, the Zeppelin danger nullified, counter measures to meet the new menace were gradually evolved. New squadrons were raised and the number of home defence squadrons was raised to fourteen service and eight night training squadrons; a Northern Home Defence Wing was formed at York; and the Home Defence Group became the 6th Brigade. The first night aeroplane raid occurred in September, and the systematic training of night-fighting pilots on scout machines was hurried on. Separate zones for aeroplanes, guns and searchlights--the latter provided with sound locators--forming an outer barrage, were instituted, and aprons, supported by kite-balloons, formed a protective barrage up to 8,000 feet. A system of wireless and ground telephonic communication was improvised for plotting the course of attacking aircraft and thus enabling squadron commanders to concentrate machines at the point of attack. By 1918 the night-fighting aeroplane, assisted by these means, had countered the night-bombing aeroplane. At first, this had been the result of the retention of a large number of fighting aircraft and a complete organization at home. Meanwhile, night fighting, especially the protection of night bombers by fighting machines, had become of paramount importance on the Western Front. The chief feature of activity in September, 1918, was the successful co-operation between searchlights in the forward areas and No. 151 night-fighting squadron. This was the first night-fighting squadron, trained by the 6th Brigade, to be sent to France. It was proposed to send four more such squadrons and thus form a first line of offensive defence which would react on hostile raids over England. Thus once again the old doctrine was gradually observed that offence is the only true defence, and that purely defensive measures, however efficient, by keeping men and material from the vital point, are necessarily expensive out of all proportion to their effectiveness. Both the Germans and ourselves made the initial mistake of organizing large local defence systems partly to placate public opinion. During the German offensive of 1918 a further development of night fighting took place in the bombing and low strafing of enemy troops and unlighted transport with the aid of flares. THE MACHINE AND ENGINE.