Not only would the velocity become so reduced as to render a “hit” capable of but little mischief, but the time of flight of the bullet, rising vertically to this altitude, would be about eight or nine seconds, and the distance moved by the aëroplane 1,000 feet, more or less. Therefore, it would be necessary to fire into quite a different part of the heavens from that in which the aëroplane was seen. The vertical range of aircraft artillery is much higher. In the case of a one-pounder having the same velocity the range would be over 12,000 feet; but it is a question of luck whether the aëroplane would be hit. The great difficulty is the angle of “lead” which must be given to allow for the velocity of flight. This angle is only constant so long as the velocity of the projectile is constant, assuming (as fairly represents the conditions) the flight speed not to vary; at extreme heights the velocity of the projectile has fallen so low that a very slight error in range-finding will be fatal to accuracy. In regard to aëroplane artillery, Mr. W. F. Reid has collected some interesting details of the guns that Krupp has devised for the purpose of hitting aëroplanes. The 7.5 cm. gun of this firm has seats for five men and storage for sixty-two shells. It is mounted on a car which weighs 4,300 kilos., the weight of the gun alone being 1,065 kilos. Each projectile weighs 5.5 kilos. (12 lb. 2 oz.), and the horizontal range is given as 9 km. The vertical range is 6,300 metres. A lighter gun of 6.5 cm. gauge weighs, with car, 875 kilos., the gun weighing 352 kilos. Each projectile weighs 4 kilos. (8 lb. 13 oz.), and the extreme horizontal range is stated to be 8,650 metres (9,450 yards). The height of fire obtainable is 5,700 metres (18,700 feet). The initial velocity of the projectile is 620 metres (2,034 feet) per second. A coiled spring balances the weight of the gun when pointed above the horizontal. For naval purposes Krupp has constructed a 10.5 cm. gun weighing 3,000 kilos, with carriage. The projectile weighs 18 kilos. (40 lb.). The muzzle velocity is 2,100 feet per second, and the shells discharge a train of smoke to facilitate aiming. Ehrhardt, in Düsseldorf, has also built a special gun for use against aërial craft. Its bore is 5 cm., and its barrel is 30 calibres long, while the length of the Krupp barrels is 35 calibres. The weight of the Ehrhardt gun alone is 400 kilos.; with car, ammunition, and five men the weight is 3,200 kilos. With regard to the difficult subject of armouring aëroplanes, I should like again to quote from Mr. Lanchester:--