In regard to the stability of the machine, the centre of weight is much below the centre of lifting effect; moreover, the upper wings are set at such an angle that whenever the machine tilts to the right or to the left the lifting effect is increased on the lower side and diminished on the higher side. This simple arrangement makes it automatic as far as rolling is concerned. I am of the opinion that whenever flying machines come into use, it will be necessary to steer in a vertical direction by means of an automatic steering gear controlled by a gyroscope. It will certainly not be more difficult to manœuvre and steer such machines than it is to control completely submerged torpedoes. When the machine is once perfected, it will not require a railway track to enable it to get the necessary velocity to rise. A short run over a moderately level field will suffice. As far as landing is concerned, the aerial navigator will touch the ground when moving forward, and the machine will be brought to a state of rest by sliding on the ground for a short distance. In this manner very little shock will result, whereas if the machine is stopped in the air and allowed to fall directly to the earth without advancing, the shock, although not strong enough to be dangerous to life or limb, might be sufficient to disarrange or injure the machinery. THE COMPARATIVE VALUE OF DIFFERENT MOTORS. So far I have only discussed the navigation of the air by the use of propellers driven by a steam engine. The engines that I employ are what is known as compound engines--that is, they have a large and a small cylinder. Steam at a very high pressure enters the high-pressure cylinder, expands and escapes at a lower pressure into a larger cylinder where it again expands and does more work. A compound engine is more economical in steam than a simple engine, and therefore requires a smaller boiler to develop the same horse-power, so that when we consider the weight of water and fuel for a given time, together with the weight of the boiler and the engine, the engine motor with a compound engine is lighter than a simple engine. However, if only the weight of the engine is to be considered then the simple engine will develop more power per unit of weight than the compound engine. For instance, if, instead of allowing the steam to enter the small cylinder, and the exhaust from this cylinder to enter the large or low-pressure cylinder--which necessitates that the high-pressure piston has to work against a back pressure equal to the full pressure on the low-pressure cylinder--I should connect both cylinders direct with the live steam, and allow both to discharge their exhaust directly into the air, I should then have a pair of simple engines, and instead of developing 363 H.P. they would develop fully 500 H.P., or nearly 1 H.P. for every pound of their weight. I mention this fact to show that the engines are exceedingly