The manner in which the natural wing (and the artificial wing properly constructed and propelled) evades the resistance of the air during the up stroke, and gives continuous support and propulsion, is very remarkable. Fig. 115 illustrates the true principle. Let _a b_ represent the horizon; _m n_ the direction of vibration; _x s_ the wing ready to make the down stroke, and _x t_ the wing ready to make the up stroke. When the wing _x s_ descends, the posterior margin (_s_) is screwed _downwards_ and _forwards_ in the direction _s_, _t_; the forward angle which it makes with the horizon increasing as the wing descends (compare with fig. 85 (_a b c_), p. 160, and fig. 88 (_c d e f_), p. 166). The air is thus seized by a great variety of inclined surfaces, and as the under surface of the wing, which is a true kite, looks _upwards_ and _forwards_, it tends to carry the body of the bird _upwards_ and _forwards_ in the direction _x w_. When the wing _x t_ makes the _up stroke_, it rotates in the direction _t s_ to prepare for the second down stroke. It does not, however, ascend in the direction _t s_. On the contrary, it darts up like a true kite, which it is, in the direction _x v_, in virtue of the reaction of the air, and because the body of the bird, to which it is attached, has a forward motion communicated to it by the wing during the down stroke (compare with _g h i_ of fig. 88, p. 166). The resultant of the forces acting in the directions _x v_ and _x b_, is one acting in the direction _x w_, and if allowance be made for the operation of gravity, the flight of the bird will correspond to a line somewhere between _w_ and _b_, probably the line _x r_. This result is produced by the wing acting as an eccentric--by the upper concave surface of the pinion being always directed upwards, the under concave surface downwards--by the under surface, which is a true kite, darting forward in wave curves both during the down and up strokes, and never making a backward angle with the horizon (fig. 88, p. 166); and lastly, by the wing employing the air under it as a fulcrum during the down stroke, the air, on its own part, reacting on the under surface of the pinion, and when the proper time arrives, contributing to the elevation of the wing.