

230 DR. PETTIGREW ON THE MECHANISM OF FLIGHT. 



angle, and consequently always strike at the same degree of obliquity. Here, again, 

 power is lost, the two vanes striking after each other in the same manner, in the same 

 direction, and almost at precisely the same moment, — no provision being made for 

 increasing the angle, and the propelling-power, at one stage of the stroke, and reducing 

 it at another, to diminish the amount of slip incidental to the arrangement. This result 

 is obtained in marvellous perfection in the wings, and by a very simple contrivance, the 

 angle which the pinions make with the horizon being gradually increased by the wings 

 rotating on their long axes during the down or effective stroke, as it is usually termed, 

 to increase the elevating- and propelling-power, and gradually decreased during the up or 

 non-effective one to reduce the resistance occasioned by their ascent and backward move- 

 ment, while it actually increases the sustaining area by placing the wing in a more hori- 

 zontal position. It follows from this arrangement that every particle of air within the 

 wide range of the wings is separately influenced by them, both during their ascent and 

 descent, — the elevating-, propelling-, and sustaining-power being by this means increased 

 to the utmost, while the slip or waftage is reduced to an infinitesimal and almost nominal 

 amount. The effect aimed at is further secured by the undulatory or wave-like track de- 



- 



scribed by the wing during the down and up strokes (Diagram 6, p. 233) ; and it is a some- 

 what remarkable circumstance that the wing, when not actually engaged as a propeller and 

 elevator, acts as a sustainer after the manner of a parachute. This it can readily do, alike 

 from its form and the mode of its application, the double curve or spiral into which it is 

 thrown in action enabling it to lay hold of the air with avidity, in whatever direction it is 

 urged. I say " in whatever direction," because, even when it is being recovered or drawn 

 off the wind during the back stroke, it is climbing a gradient which arches above the 



© " © 



body to be elevated, and so prevents it from falling. It is difficult to conceive a more 

 admirable, simple, or effective arrangement, or one which would more thoroughly econo- 

 mize power. Indeed, a study of the spiral configuration of the wing, and its spiral, flail- 

 like, lashing movements, involves some of the most profound problems in mathematics, 

 the curves formed by the pinion as a pinion anatomically, and by the pinion in action, 

 or physiologically, being the most elegant and precise which it is possible to imagine, 

 these running into each other, and merging and blending, to consummate the triple func- 

 tion of elevating, propelling, and sustaining. If further proof were necessary, it would 

 be found in the fact that, during the down or effective stroke, the anterior and under 

 extremity of the tip of the wing (Plate XV. fig. 62/) lays hold of the air with a biting 

 or concave surface in a direction from above downwards, from behind forwards, and 

 from without inwards, and forces it along a spiral groove on the under surface of the 



wing (vide arrows of figure) to the root of the same (g), where it causes it to escape by 

 a convex one. 



The under or ventral surface of the pinion is therefore engaged in elevating, pro 



pellmg, and sustaining in a compound sense, one portion (the tip or outer part) 

 scaling or climbing upwards and onwards, the root or inner portion aiding and abet- 

 ting by pushing in a similar direction from beneath. In the return or back stroke, as 

 has been explained, the curves formed by the under surface of the wing are reduced 

 in such a manner as to decrease the amount of friction, while they increase, rather 











