names of all the known motors, and still less of the apparatuses which might be applied as motors. Inventors reserve many surprises in that matter. But, without letting imagination carry us beyond the domain of experimental science, it is allowable for us to consider what satisfaction steam, electricity, and such accumulators of energy as India rubber, steel, compressed air, gas motors, and explosives may give. We are able now, with special precautions, to construct steam motors of extreme levity, and giving one horse-power for a weight very near that of 3·5 kilogrammes; but if we add to them the indispensable generator and the inevitable propeller, the weight increases in formidable proportions, and the system becomes inapplicable to any mode of support in the air.

Electricity, although it is better in many respects, is likewise liable to criticism. Yet we had the honor of performing some satisfactory experiments with it in 1887 at the Scientific Congress in Toulouse, and in 1888 at the Easter session of the Société de Physique. We had taken all possible care in the construction of a motor; it was all of aluminum, with the exception of the poles, which were of soft iron. Its weight was ninety grammes, and its power, measured with our dynamometer, was maintained at two kilogrammetres, corresponding exactly with one horse-power per 3·375 kilogrammes. This motor, armed with a light and geometrically Fig. 2.—Electric Helicopter and Aëroplane. perfect helix, made according to a new method which we had explained to the Academy of Sciences on the 12th of July, 1886, was placed in one of the plates of a balance, and put in connection, with a constant electrical source of forty watts, when it raised its whole weight. In order to render more visible the extent of the result, and obtain a more exact idea of it, I arranged a light balance with long arms, to one of which I attached the motor experimented on, as in Fig. 2. The electric communications, carried through the foot, knife-edges, and arms of the balance, can not obstruct the freedom of its motion. Being movable in the vertical and horizontal directions, the balance changes immediately from the position A B to that of A' B'. The power developed by the motor is found, by the most careful measurement, equivalent to two kilogrammes—a power so related to the weight of the motor as to be capable of raising it vertically twenty-two