Page:Popular Science Monthly Volume 40.djvu/410

394 the motor. It is not till that is got that calculation should come in, when it can find a sure starting-point, based on well-conducted experiments and precise ideas, and its results will be susceptible of an immediate verification. We have constantly employed this prudent, positive method, and it only can give satisfactory results. This motor, which is to fulfill at the same time the two conditions so hard to reconcile, of great power and extreme levity, we shall now try to describe.



The fact indisputably results from observations, from the positive experiments of M. Marey, from the studies of M. Espitalier, and from our personal labors, that birds expend on an average a motor exertion of 75 kilogrammes per unity of weight—a unity comprehended between 3·6 and 12·5 kilogrammes—in rising vertically one metre per second. Observe that we are talking of gross work, not of useful work effected directly upon the air. Thus Goupil, a respected authority, has found that the work of a horsepower in the pigeon is given for a weight of 12·5 kilogrammes. That is the manifest work, but not the work really developed by the animal; the wing, like the screw, in fact, makes only a weak return.

We select, then, the minimum unity of weight 3·5 kilogrammes per horse-power which results from the experiment with our electrical helicopter, because we know in advance that we can not obtain the full return for the expenditure; and in this weight we must include that of the generator of energy, or of the propeller, and all the accessories.

It is impossible, in this necessarily brief study, to give the