228 DR. PETTIGREW ON THE MECHANISM OF FLIGHT. 



the air with such dexterity as to rob flight to a great extent of its inequalities, by con- 

 verting it into a more or less perfect gliding movement (Plate XV. fig. 59 a a ; Diagram 6, 

 p. 233). The down and up strokes, being essentially different and opposite movements, 

 tend likewise to equalize each other. The gliding referred to is occasioned by the gradual 

 extension and depression of the wing, during the down stroke, and its gradual flexion and 

 elevation during the return or up stroke, the one movement, in fact, gliding into the other. 

 The wing in action consequently describes a spiral course from within outwards andjfrom 

 above downwards during extension and the effective or down stroke, and from without in- 

 wards and from below upwards during flexion and the back or up stroke. The result of 

 this is, that the wing is piercing the wind in two directions at nearly the same instant of 

 time, — the interval which is consumed in extending and flexing the pinion being dedicated 

 principally to applying it to and withdrawing it from the wind, that which elapses during 

 the down and up strokes to urging the animal upwards and forwards. That the wing rotates 

 upon its long axis, as explained, may be readily ascertained by watching the movement in 

 the larger domestic fly. If the wing be viewed during its vibrations from above, it will be 

 found that the blur, or impression produced on the eye by its action, is more or less con- 







(the under surface of the blur being convex). This is due to the fact that the 



•o 



is spiral in its nature (Plate XV. fig. 68), because, during its action, it twists upon 

 itself in such a manner as to describe a double curve (Plate XV. fig. 58 a a' ; com- 

 pare with a a of fig. 52), the one curve being directed upwards (a), the other down- 

 wards (a). The double curve or wave-track referred to is particularly evident in the 

 flight of birds, from the greater size of their wings (Plate XV. figs. 73 and 75 b a c)*. 

 It may not inaptly be compared to the blade of an ordinary screw propeller, as employed 

 in navigation (Plate XV. fig. 52 a a'). The effect obtained, moreover, is in some respects 

 analogous, since the great velocity with which the wing is driven converts the impression 

 or blur (Plate XV. figs. 58 and 59) into what is equivalent to a solid for the time being, 

 in the same way that the spokes of a wheel in violent motion, as is well understood, 

 completely occupy the space contained within the rim or circumference of the wheel. 

 The following differences are to be noted ; and they are of importance, as they seem to 

 prove very satisfactorily the immense superiority of the wing over every form of screw 

 propeller yet constructed : 



Points toherein the screws formed by the wings differ from those in common use.— I. In 

 the blade of the ordinary screw (Plate XV. fig. 52) the integral parts are rigid and unyield- 

 ing, whereas m the blade of the screw formed by the wing (Plate XV. figs. 68, 69, and 70), 

 and m the twisted blur produced by its action (Plate XV. figs. 58, 59, and 61), they are mo- 

 bile and plastic. This is a curious and interesting point, the more especially as it does 

 not seem to be either appreciated or understood. The mobility and plasticity of the wing 

 is necessary because the pinion is an elevating and sustaining organ, as well as a pro- 



petting one. 







His Grace the Duke of Argyll is of opinion that the curvature of the wing in birds is owing to the elasticity and 

 bending upward, of the tips of the primary, secondary, and tertiary feathers during the down stroke. It, however, also 

 occurs m the wings of insects and bats, which are devoid of feathers ; so that I am inclined to refer it, as stated, to the 

 conformation of the wing and to its peculiar mode of action. 



