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.