DR. PETTIGREW ON THE MECHANISM OF FLIGHT. 2M 



than diminish, the extent of sustaining area, the back or upper convex surface of the 

 pinion being turned in the direction in which the wing travels during its ascent. 



5. In the ordinary screw propeller of commerce, the bilge or backwater of the one blade 

 is urged towards and interferes with the action of the reciprocating and opposite blade; 

 whereas in the wings, which are of necessity separated by the body which bears them, 

 there is no such hindrance, and in fact no impediment whatever, each blade, ft. e. each wing 

 (Plate XIII. figs. 15 and 19; Plate XIV. figs. 31, 32, and 12) being free to utilize to the 

 utmost the large subsidies of air on which it depends for support and progress. 



6. The axis of revolution in the ordinary screw corresponds to the plane of progression 

 (Plate XV. fig. 52). The axes of the wings, on the other hand, are at right angles to it 

 (Plate XV. fig. 60). The wings may therefore be said to combine, during their action, 

 the grasp and steadiness of the paddle with the easy subtle gliding motion peculiar to 



the helix. 



Other differences might be pointed out ; but the foregoing embrace the more funda- 

 mental and striking. Enough, moreover, has probably been said to show that it is to 

 wing- structures and wing-movements the aeronaut must direct his attention, if lie would 

 learn "the way of an eagle in the air," and if he would rise upon the whirlwind in 



accordance with natural laws. 



The Wing a twisted Lever or ILelix-Ats peculiar screwing Action in the Insect, Bird, $0. 



The twisting screw-like action of the wing on its long axis during the up and down 

 strokes, as well as the range of the pinion (as seen in the Blow-fly) is given at figures 58 

 and 59 of Plate XV. It may be readily witnessed by fixing the insect and holding it, 

 with its head directed towards the spectator, against a dark background. The twisting 

 in question is most marked in the posterior or thin margin of the wing, the anterior 

 and thicker margin performing more the part of an axis. As a result of this arra™ 

 ment, the anterior or thick margin cuts into the air quietly, and as it were by stealth, 

 the posterior one producing on all occasions a violent commotion, especially perceptible 

 if a flame be exposed behind the insect. That the wing twists upon itself struc urally 

 not only in the insect, but also in the bat and bird, any one may readdy sat isfy himself 

 by a careful examination (Plate XV. figs. 68, 69, and 70) ; and hat it twists apon i sdf 



during its action I have had the most convincing and repeated proofe (Hate X . fig* 



58, 61° 73, and 74). Indeed it is matter for surprise that the spira conformation of 

 the pmion and its spiral mode of action, should have eluded f^^^T""^ 

 I shall be pardoned for dilating upon the subject when I state my «"*"»** 

 forms the fundamental and distinguishing feature in flight, and ^^^ 

 account by all those who seek to solve this most involved and interesting problem by aiti- 



o 



and 



ficial m 



* r u , » > ifiiiJE'l rtPvoted a lai-e share of time and attention to wings and wing-structures, and to the 



ff n; ar r Ids S - I propounded and s^. „ «-« of the ** configuration and 



flight of wild and tame birds In h s year a P P ^ ^ the dowu and upstroke _the 



screwing action of ^V^^J^^ bv a twisting, screwing, or semirotatory movement emanatmg 

 wing being, in my estimation, a true ACM. wo j t, ^^ ^ ^ 



principally from the shoulder, but al, ^^^JE^ ££ on uie sullj ect at the Royal Institution 

 tinotlv iminl in thpir nature. In August I860, 1 undertook w uti 



tinctly spiral in their nature. 



