While the single-seater tractor was developing for purely offensive action, the two-seater fighter, of which the field of view, manœuvrability and general performance were being improved, retained its utility as a reconnaissance machine. In 1916 the "pusher" type was superseded by the Sopwith "1½ Strutter" armed with a synchronized Vickers gun, which for its 130 horse-power was never surpassed. The pilot was close to the engine and had a good view of the ground, while the gunner was placed behind him with a rotary Lewis gun turret. Early in 1917 these qualities were further developed in the Bristol Fighter.

With the advent of these improved types the B.E.2c was relegated to the work of artillery co-operation, until superseded by the B.E.2e. Towards the end of 1916 appeared the R.E.8 with a Vickers synchronized gun and a Lewis gun, which after many vicissitudes became the standard machine for artillery work.

Systematic bombing was practised by nearly all types of machines, but real accuracy was never obtained. Thus, the B.E.2c was first used in formations, but with a full load of bombs it could not carry an observer, and its moderate speed left it an easy prey to hostile fighters. Early in 1916 appeared the Martinsyde single-seater bomber with an endurance of 4½ hours, and in 1917 the D.H.4 which was much used for day-bombing. The F.E.2b pusher, discarded as a fighting machine, became the principal night-bomber.

It was comparatively late in the war before special bombing machines were evolved. They were then divided into day-bombers and night-bombers, the D.H.9 and 9a machines being typical of the former and the Handley Page of 1917—a large twin-engine aeroplane, the first really effective night-bomber, of considerable carrying power but low performance—of the latter. By November 8th, 1918, two super-Handley Pages were ready to start to Berlin. They possessed a maximum range of 1,100 miles, a crew of seven, four 350 horse-power Rolls-Royce engines, arranged in pairs, a tractor and a pusher in tandem on either side of the machine, and, as they would be compelled to fly both by night and day, a gun defence system. The D.H.10a and the Vickers Vimy, for day and night bombing respectively, were also being produced at the date of the Armistice.

In the early days of the war an aeroplane had little to fear above 4,000 feet. With the improvement of the anti-aircraft gun there was, by the end of the war, no immunity at 20,000 feet. Very low flying for attack was, however, being rapidly developed, and would have proved of great effect in 1919. The aeroplane used for this purpose was the single-seater fighter, and the Sopwith "Salamander," with two guns, a speed of 125 miles an hour, and 650 lb. of armoured plates, was about to make its appearance at the Armistice.

I have previously mentioned how dependent the improvement of design and performance of aircraft has been upon the less simple and tardier development of the engine. The invention of the light motor made aviation possible, and development has synchronized with the evolution of lighter, more powerful and more reliable engines. One of the most difficult problems still confronting us is the production of a cheap, high-powered and reliable engine, but the existence at the end of the war of machines weighing 15 tons indicates the progress achieved, while British engines of 600 horse-power are now in use, and one of 1,000 horse-power will shortly be available.

Tactics and the Strategic Air Offensive.

During the war there were three concurrent movements in process: the ratios of the various forms of air tactics were constantly changing, and the components of our air forces varied in accordance with the development of reconnaissance, artillery co-operation, bombing and fighting. Secondly, their total strength was increasing rapidly; and, thirdly, it was increasing relatively faster than the Army or Navy.

It was an evident and logical development and in accord with the shortage of national man power and the consequent tendency to a reduction in the strength of the Army, that, the necessary uses of aircraft with the Army and Navy being ensured, any available margin of air power should be employed on an independent basis for definite strategic purposes. The difficulty was to arrive at an agreement as to the minimum tactical and grand tactical requirements of the Army and Navy. The British Army was not alone in asserting that there was no minimum and that it wanted every available airman, and agreed with the French that anything which it could temporarily spare should be lent to the French Army. It was argued that the Armies could as easily and better arrange for strategic bombing. Fortunately in 1918, when I was Chief of the Air Staff, we managed to secure a margin and formed the Independent Air Force in June of that year. It was, of course, understood that, in the event of either the British or French Armies being hard put to it, the Independent Air Force could temporarily come to their direct assistance and act in close co-operation with them.

In 1915 in accordance with the old doctrine that offence is the best defence, the surest method of protecting specialized machines on the battle front was found to be in the attack of enemy aircraft by fighting machines. In 1918 it was decided that raids on the centres of German war industry would not only cripple the enemy's output of material essential to victory, but also relieve the pressure on the Western Front, the vital point of the war. The Germans had had the same intention in the many raids which started over Dover on December 21st, 1914.