IV. Power-plant of aeroplanes—Fitting two engines to obviate involuntary descents. While writing of aeroplane development, mention should certainly be made of engines. It has been shown, already, what a wonderful step forward was made with the introduction of the revolving "Gnome." This engine will operate with the reliability of that fitted to a motor-car; there are other types of engine now achieving thoroughly good results; and they promise to do better in the future. From the point of view of propulsion, therefore, the future of the aeroplane appears assured. One great objection of sceptics, in regard to the power-plant of aeroplanes, has been that, should an engine fail, a machine is compelled to make an involuntary descent. In answer to this criticism, advocates of the aeroplane have pointed out that motors are becoming more and more reliable, and that such stoppages are already reduced to a minimum. It is now seen that it will be possible to prevent involuntary descent, when flying across country, by equipping aeroplanes with a reserve powerplant. One of the most interesting experiments, in this regard, is being carried out by Mr Horace Short, the aeronautical engineer of the Royal Aero Club. Mr Short produced, in September, 1911, a biplane fitted with two "Gnome" engines. One, placed in front of the machine, operated a couple of tractor propellers fitted before the main-planes. The other, installed at the rear, actuated a single propeller. The machine was designed so that one engine would drive it at a speed of about thirty-six miles an hour; while, if both were employed, its pace would increase to more than fifty miles an hour. In actual tests, most encouraging results were obtained with this machine, and Mr Short is now perfecting this type of twin-engine biplane. By equipping an aeroplane with two engines, working independently of each other, the danger of an involuntary descent, on bad ground, is obviated. Should one engine stop, the pilot can keep himself in the air with the other. In such a machine as this, which might be described as a "safety" aeroplane, the weight of the additional engine is, of course, a consideration, but in cases where exceptional reliability is required—say, for example, when a most urgent dispatch has to be taken across country from point to point in military operations—a dual-engine machine will probably be used. Consideration of such facts as have been cited in this section show that aeroplanes, for military work, have an almost unlimited field for improvement. Therefore, any nation which neglects war machines now, on the argument that they are unreliable, is pursuing an extremely foolish policy. If feats such as have been recently accomplished are possible, with machines built with wood, wire, and canvas, what will not be achieved with the stronger, speedier, heavier aeroplanes, such as are already being constructed?