In 1866 Mr. Wenham pointed out that birds whilst in rapid flight encountered at each instant a fresh undisturbed body of air, and dispensed less power in full flight than in hovering. M. de Lauvire showed also, about the same time, the advantage of the oblique action of surfaces upon the air, in taking for basis the experiments of Thibault.

I have been able to establish the very simple law of the resistance to flat surfaces moving at very oblique angles in a fluid, and I developed their result in 1872.

By introducing in my calculations the results of several observations that I have made upon the different species of birds, I determined very nearly the work dispensed by them in full flight, this work was equivalent according to the species and the size of wings, to the elevation of the weight of the animal from 20in. to 5ft. per second, and generally superior to 40in. for the large species.

Newton, who was the first to study the resistance that fluids offer to a body moving in them, stated implicitly that the molecules of the fluid remained immovable up to the moment that the body touched them, and returned to a state of rest immediately afterwards.

He found that the resistance experienced by a flat surface was proportionate, 1° to its extent, 2° to the density of the fluid, 3° to the square of the velocity, 4° to the square of the sine of the angle of incidence, and 5° that it is normal to the surface. It was, however, soon discovered that this theory, altogether empirical, was often at discord with what experience taught, and a great number of experimental researches have been made at different periods in order to throw further light on the subject.

Since the days of Bishop Wilkins the scheme of flying by artificial wings has been much ridiculed, and indeed the idea of attaching wings to the arms of a man is ridiculous enough, as the pectoral muscles of a bird occupy more than two-thirds of its whole muscular strength, whereas in man the muscles that could operate upon the. wings thus attached would probably not exceed one-tenth of the whole mass. There is no proof that, weight for weight, a man is comparatively weaker than a bird; it is therefore probable, if he can be made to exert his whole strength advantageously upon a light surface similarly proportioned to his weight, as that of the wing to the bird, that he would fly like a bird. The flight of a strong man by great muscular exertion, though a curious and interesting circumstance, inasmuch as it will probably be the first means of ascertaining this power and supplying the basis whereon to improve, it would be of little use.