[70] In birds which skim, sail, or glide, the pinion is greatly elongated or ribbon-shaped, and the weight of the body is made to operate upon the inclined planes formed by the wings, in such a manner that the bird when it has once got fairly under weigh, is in a measure self-supporting. This is especially the case when it is proceeding against a slight breeze--the wind and the inclined planes resulting from the upward inclination of the wings reacting upon each other, with this very remarkable result, that the mass of the bird moves steadily forwards in a more or less horizontal direction. [Illustration: FIG. 59.--The Red-legged Partridge (_Perdix rubra_) with wings fully extended as in rapid flight, shows deeply concave form of the wings, how the primary and secondary feathers overlap and support each other during extension, and how the anterior or thick margins of the wings are directed upwards and forwards, and the posterior or thin ones downwards and backwards. The wings in the partridge are wielded with immense velocity and power. This is necessary because of their small size as compared with the great dimensions and weight of the body. If a horizontal line be drawn across the feet (_a_, _e_) to represent the horizon, and another from the tip of the tail (_a_) to the root of the wing (_d_), the angle at which the wing strikes the air is given. The body and wings when taken together form a kite. The wings in the partridge are rounded and broad. Compare with heron, fig. 60.--_Original._] [Illustration: FIG. 60.--The Grey Heron (_Ardea cinerea_) in full flight. In the heron the wings are deeply concave, and unusually large as compared with the size of the bird. The result is that the wings are moved very leisurely, with a slow, heavy, and almost solemn beat. The heron figured weighed under 3 lbs.; and the expanse of wing was considerably greater than that of a wild goose which weighed over 9 lbs. Flight is consequently more a question of power and weight than of buoyancy and surface. _d_, _e_, _f_ Anterior thick strong margin of right wing. _c_, _a_, _b_ Posterior thin flexible margin, composed of primary (_b_), secondary (_a_), and tertiary (_c_) feathers. Compare with partridge, fig. 59.--_Original._]