And a third difficulty was that of starting the machine quickly in pursuit of an enemy travelling at the great heights that are now customary—and indeed obligatory—with the development of anti-aircraft artillery. Naturally the machine even of the speedier sort loses time as it mounts spirally or in a series of inclined planes to give battles to the Zeppelin or Fokker. Would it be possible to turn the observation balloon into a sort of perch for the airman, so that he would be suspended always midway between earth and sky ready instantly (if one may suppose him able to detach himself) to fly away in pursuit of the stranger? Yet in the present stage of development such a desideratum is difficult of realisation. The airman must start from the ground or return to it every time he wants to overhaul his engine or replenish his reservoirs. That is his touch with solid realities: otherwise one might suppose him flying for days, never setting foot to earth, the modern Guardian Angel, hovering eternally in the heavens. The aeroplane has completely revolutionised warfare, inasmuch as it has deprived strategy of its chief weapon—surprise. As the eyes of the army the aeroplane played its most important rôle. A light and very speedy machine is the scout, and it is his duty to make reconnaissances, report upon gun emplacements, the numbers of the opposing troops, the movements of the enemy, and the disposition of his trenches. But the aeroplanes used in warfare are not all alike. The tendency is towards differentiation, and while the scout is swift and light, the battle-plane is extremely powerful and heavily armed with cannon or machine-guns, sometimes also carrying a spur for ramming enemy craft; and again there is a third type of machine armed also for defence, but adapted principally to range-finding, and fitted with signalling and photographic apparatus; it hovers continually over the enemy lines directing artillery fire. None of the offices of the aeroplane proved more valuable than that of giving the range of enemy positions to one’s own artillery, and then registering the shots, marking where they fell too short, or overpassed the mark. This is one of the most dangerous as well as the most useful of the services rendered by the man-bird. It requires great nerve, judgment, and coolness on the part of the aviator, for he must hang over the enemy trenches and expose himself to the fire of their anti-aircraft guns, the efficacity of which made rapid strides as the war progressed. The German method of signalling to the opposing batteries by means of smoke bombs with different-coloured fuses was soon improved upon by the French, who used wireless telegraphy and the heliograph by day. But the rôle of the aeroplane is not confined even to these important services. It becomes at times the instrument of aerial bombardment for the destruction of fortified places, military stores, railway junctions, dirigible sheds, encampments, bridges, and