metres in a second. The simple theoretical calculation deduced from the experimental fact assigns 3·375 kilogrammes to the motor that will develop seventy-five kilogrammes. But so minute a motor returns only about twenty per cent of the energy which is confided to it, while a motor of from fifty to one hundred horsepower will return eighty, ninety, or one hundred per cent. It is possible, therefore, and seems to be reasonable, that a large electric motor, the power of which increases faster than the weight, would employ the surplus of sixty or seventy per cent in raising the generator, the propeller, and the aëronaut. We do not intend to hypothecate the future and form tables on gratuitous suppositions, probable as they may seem. We therefore, for the moment, lay aside the electric motor, because, with its generator and propeller, it exceeds the weight of 3·5 kilogrammes per horse-power, which we have imposed upon ourselves as the minimum.
We now come to accumulators of energy. India rubber, for example, the elasticity of which is often utilized as a reservoir of power, and has a potential, in this point of view, fifteen times superior to that of steel, furnishes power and motion together. Joining to it an immediate organ of resistance to the air, we have an apparatus heavier than air. Penaud chose admirably; and one of the first helicopters was formed upon this plan. But, while India rubber stores a large sum of energy, it expends it faster than it obtains it, and can not of itself renew the provision. Penaud had only a small success with it, because the thongs he used were placed and displaced too slowly; and if he had found a means of changing them more rapidly, the considerable charge of his provision would have made him lose the primary advantages of his judicious choice.
Compressed air motors and gas motors enjoy a certain repute which is in many respects deserved; but as they are constructed, they require the assistance of lubricating and refrigerating apparatus which have weight, and are thus excluded for the present from the list of applications for aerial locomotion. So there are no steam motors, or electric motors, or accumulators of energy like India rubber, or steel, or compressed air motors or generators, that fully answer the requirements. None of them, as they are, supply such coexisting conditions of power and levity as are strictly imposed by the nature of the problem. Is it, then, true that there is now no motor with its accessories, the generator and propeller, which can be used at once, or at least improved upon, for the purpose we have in view? The comparative experiments which we have reported, and have verified with our new universal direct-reading dynamometer, which we had the honor of presenting to the Academy on the 23d of June, 1891, seem to attest this. Still, if the generator and propeller, mutually necessary.
