. Animal flight; a record of observation . or when soaring Fig. of retiring wings. Let us first consider the case of flapping flight. Wehave already seen that if the bird while flapping changesits wings from the straight to the retired positionit rotates round its transverse axis and the direction ofits flight is in a downward direction. Conversely ifas in stop-flapping the bird advances its wings itrotates upwards round the transverse axis. These facts 1 ^ Fig. 59. Outline of a vulture circling in air notfully soarable, or circling in fully soarableair without effort to gain height.
. Animal flight; a record of observation . or when soaring Fig. of retiring wings. Let us first consider the case of flapping flight. Wehave already seen that if the bird while flapping changesits wings from the straight to the retired positionit rotates round its transverse axis and the direction ofits flight is in a downward direction. Conversely ifas in stop-flapping the bird advances its wings itrotates upwards round the transverse axis. These facts 1 ^ Fig. 59. Outline of a vulture circling in air notfully soarable, or circling in fully soarableair without effort to gain height. 196 ANIMAL FLIGHT. suggest that the law holds good. But if this is the casewhy is it that the wings are advanced in slow horizontalflight, and why does the amount of advancing diminishas speed increases ? Supposing a birdis gliding horizon-tally in calm air,and someone mo-mentarily catcheshold of its tail soas to check speed ahead. Supposing in consequencethe bird were to flap its wings up and down in order toregain speed ahead. Then at
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