An examination of the wings of birds, whether those of soarers or of any other type, impresses one not only with the general strength of the wing, but also with the fact that, while it possesses considerable stiffness, there is also a graduated pliability, not only of the whole wing, including the bones, but more especially in the feathers, the rear tips being exceedingly pliable so that, when the wing is held in a stiff breeze, they are seen to be easily deflected in a gentle curve towards the rear and upper side. This lack of rigidity has several advantages, among the more notable of which is the lessening of the strains on the wing caused by sudden wind gusts. Of great importance is the further fact that a supporting surface having a graduated pliability, such as is possessed by a bird’s wing, does not experience a shifting of the center of pressure to the same extent as a rigid surface of similar form. Furthermore, since any bird, even the best soarer, must use its wings not only for soaring, but, when starting to fly from a state of rest, for flapping, a rigid surface would not furnish anything like the same universally available sustaining and propelling means that the bird’s wing does. In an inspection of the various wings or supporting surfaces which Mr. Langley built, from the very earliest rubber-pull models up to the successful steam machines Nos. 5 and 6, the point which is most impressed upon the observer is the increasing strength and rigidity embodied in these wings. While the success with the later models was due to many things, including the development of a strong frame and a suitable power plant giving sufficient power for the permissible weight, besides the very important development of effective equilibrium mechanism, yet it is safe to say that even with the development of all these other things to the state to which they had been brought in 1896, success would not have been achieved had not the wings themselves been simultaneously changed from the very flimsy construction which was at first used to the later type, using a very strong and rigid wooden frame over which the cloth covering was tightly stretched, and which possessed only a small amount of pliability at the extreme rear ends of the cross-ribs. The development of this successful type of wing for the models, it will be remembered, had been achieved only after an extensive series of experiments; and it was realized that the construction of suitable wings for the large aerodrome, even with the knowledge gained in the early work, would be still more [p189] difficult. The problem was that of constructing for a very little greater weight per square foot, wings containing approximately sixteen times the area of the model wings.