PETTIGREW ON THE MECHANISM OF FLIGHT. 227 



ing at a given angle in an upward direction. In those birds, insects, and bats which 

 flap the wings leisurely, the stroke is delivered in an almost perpendicular direction, 

 the wing, by rotating on its axis from behind forwards, causing the under surface 

 of the pinion to act upon the air obliquely from above downwards and fmm behind for- 

 wards. The down stroke in slow-flying creatures is delivered nearly at right angles to 

 the body, which is inclined in a slightly upward direction. The upward inclination of 

 the body, and the comparative perpendicularity of the stroke, is necessary to counteract 

 the tendency of the slow-flying animals to fall vertically downwards. In those insects, 

 bats, and birds, however, whose wings are moved with great celerity and the speed at- 

 tained is high, the down stroke is inclined very decidedly forwards — the tendency of 

 the body to fall downwards and forwards increasing as the speed of the wing and the 

 velocity of flight is augmented. The forward movement of the wing dozing the down or 

 effective stroke is particularly evident in birds when rising, the wing, on such occa- 

 sions, being urged with unusual vigour. The forward movement of the wing during the 

 down stroke is singularly well seen in young Pigeons when thrown from the hand for 

 the first time ; and I have also noticed it in the Cormorant when leaving the water— this 

 bird, because of its great weight, rising with considerable difficulty. The forward more- 

 ment of the wine durine its descent is also seen to advantage in the flight of the wild 



b "-^^"b 



Goose and Duck, both of which fly with immense velocity. It can be increased and 

 diminished at pleasure, and assists in regulating the rapidity of flight. The wing, in 

 slow-flying insects, bats, and birds (Plate XIV. fig. 38), supports the centre of gravity 

 by playing, as it were, alternately above and beneath it ; whereas in those of rapid 

 flight the pinion plays obliquely on either side of it, as shown in figures 58, 59, and 61, 

 Plate XV. The sustaining area of the wings is greatly increased in birds, insects, and 

 bats of rapid flight ; and this is owing partly to the oblique direction of the stroke, and 

 partly to the fact that the quickly vibrating wing, practically speaking, occupies the entire 

 space marked off by the down and up strokes, in the same way that the spokes of a wheel 



in rapid motion apparently fill its area. 



Might essentially a gliding movement.-®* angle at which the wing acts most effi- 

 caciously as a propeller and elevator, as indicated by an examination of the pinion of the 

 living insect, bat, and bird when fully extended and ready to give the effective stroke, 

 is something like one of 30° with the horizon. It varies, however, as has been explained, 

 according as the animal is ascending, descending, or progressing at a high speed-the 

 anele beine least when the speed is greatest. As the angle adverted to could not 



the „««, 



be uniformly maintained without a rotatory motion which would 

 from their fixings, the wing is made to rotate on its long axis to the extent of 

 something less than a quarter of a turn in one direction during extension, and a cor- 

 responding amount in an opposite direction during flexion. Since the wing in per- 

 forming this movement, travels from the plane of least resistance till it makes the angle 

 in question, and back again, at each vibration, a certain degree of power is dissipated in 

 simply applying and withdrawing the surfaces. The movement moreover, is not quite 

 so uniform as it would be if complete rotation supervened. On this head, however, 

 there is little ground for complaint, the wing being presented to and withdrawn from 



