during its _ascent_ as well as during its _descent_. It is, in fact, a kite during both the down and up strokes. The ascent of the wing is greatly assisted by the _forward travel_, and _downward and forward fall_ of the body. This view will be readily understood by supposing, what is really the case, that the wing is more or less fixed by the air in space at the point indicated by 2 of figs. 84 and 85, p. 160; the body, the instant the wing is fixed, falling downwards and forwards in a curve, which, of course, is equivalent to placing the wing above, and, so to speak, behind the volant animal--in other words, to elevating the wing preparatory to a second down stroke, as seen at _g_ of the figures referred to (figs. 84 and 85). The ascent and descent of the wing is always very much greater than that of the body, from the fact of the pinion acting as a long lever. The peculiarity of the wing consists in its being a flexible lever which acts upon yielding fulcra (the air), the body participating in, and to a certain extent perpetuating, the movements originally produced by the pinion. The part which the body performs in flight is indicated at fig. 87. At _a_ the body is depressed, the wing being elevated and ready to make the down stroke at _b_. The wing descends in the direction _c d_, but the moment it begins to descend the body moves _upwards and forwards_ (see arrows) in a curved line to _e_. As the wing is attached to the body the wing is made gradually to assume the position _f_. The body (_e_), it will be observed, is now on a higher level than the wing (_f_); the under surface of the latter being so adjusted that it strikes upwards and forwards as a kite. It is thus that the wing sustains and propels during the up stroke. The body (_e_) now falls _downwards and forwards_ in a curved line to _g_, and in doing this it elevates or assists in elevating the wing to _j_. The pinion is a second time depressed in the direction _k l_, which has the effect of forcing the body along a waved track and in _an upward direction_ until it reaches the point _m_. The ascent of the body and the descent of the wing take place simultaneously (_m n_). The body and wing, are alternately above and beneath a given line _x x´_. [Illustration: FIG. 87.]