Figs. 82 and 83 show that when the wings are elevated (_e_, _f_, _g_ of fig. 82) the body falls (_s_ of fig. 82); and that when the wings are depressed (_h_, _i_, _j_ of fig. 83) the body is elevated (_r_ of fig. 83). Fig. 82 shows that the wings are elevated as short levers (_e_) until towards the termination of the up stroke, when they are gradually expanded (_f_, _g_) to prepare them for making the down stroke. Fig. 83 shows that the wings descend as long levers (_h_) until towards the termination of the down stroke, when they are gradually folded or flexed (_i_, _j_), to rob them of their momentum and prepare them for making the up stroke. Compare with figs. 74 and 75, p. 145. By this means the air beneath the wings is vigorously seized during the down stroke, while that above it is avoided during the up stroke. The concavo-convex form of the wings and the forward travel of the body contribute to this result. The wings, it will be observed, act as a parachute both during the up and down strokes. Compare with fig. 55, p. 112. Fig. 83 shows, in addition, the compound rotation of the wing, how it rotates upon a as a centre, with a radius _m b n_, and upon _a c b_ as a centre, with a radius _k l_. Compare with fig. 80, p. 149.--_Original._]
_The Wing ascends when the Body descends, and_ vice versâ.--As the body of the insect, bat, and bird falls forwards in a curve when the wing ascends, and is elevated in a curve when the wing descends, it follows that the trunk of the animal is urged along a waved line, as represented at 1, 2, 3, 4, 5 of fig. 81, p. 157; the waved line _a c e g i_ of the same figure giving the track made by the wing. I have distinctly seen the alternate rise and fall of the body and wing when watching the flight of the gull from the stern of a steam-boat.
The direction of the stroke in the insect, as has been already explained, is much more horizontal than in the bat or bird (compare figs. 82 and 83 with figs. 64, 65, and 66, p. 139). In either case, however, the down stroke must be delivered in a more or less forward direction. This is necessary for support and propulsion. A horizontal to-and-fro movement will elevate, and an up-and-down vertical movement propel, but an oblique forward motion is requisite for progressive flight.
