A machine-gun intended for aerial use must be absolutely reliable in operation. If a gun jams on the ground there is usually time to overhaul it or to replace it. Not so in the air. There a jam or a malfunction is almost certain to prove disastrous, if not fatal, to the gunner, who is left completely at the mercy of his adversary. An aircraft gun must also function properly in any position in which it is likely to be placed by the manœuvres of the plane. Likewise, an intensely high rate of fire is essential. For groundwork 500 shots per minute is reckoned as sufficient for the machine-gun, for a higher rate of fire would only result in several bullets hitting the same man. But a considerably higher rate of fire—up to 1,000 shots a minute, in fact—is demanded of aircraft guns, this being necessitated by the great speed at which airplanes move. The gunner, remember, can train on his target for only a few seconds, sometimes for only a fraction of a second, at a time, and it is essential, therefore, that he should have at his command the greatest possible volume of fire. Do you appreciate that, were an airplane flying parallel to, say, a high board fence, at a speed of 100 miles an hour, and shooting at right angles at that fence with a gun firing 880 shots a minute, _the bullet-marks on the fence would be ten feet apart_? Single-seater machines carry only fixed guns, which are mounted with the barrel parallel to the axis of the airplane. These guns, which are synchronized so as to shoot through the propeller, are put into action by a trigger on the “joy-stick” of the plane and are aimed by pointing the entire airplane at the enemy. Flexible guns are used only on two-place machines, being operated by the observer or gunner. They are carried on the Universal mount, which permits of the gun being pointed in any direction. All of the flexible aircraft guns used by the Allies were based on the principle of the Lewis gun, the invention of a retired American army officer, Colonel Isaac Lewis. The chief difference between the ground and aircraft models is that in the latter the cooling radiator is eliminated, as aircraft guns are never fired continuously for any length of time.