DB PETTIGREW ON THE PHYSIOLOGY OF WINGS. 35) 



is active— the air passive. In other words, the wing is depressed by a purely 

 vital act. This is proved by taking a living or dead blow-fly, and forcibly 

 depressing its wing in the direction of the head by the aid of a slender rod. 

 This act causes the wing to make various angles of inclination with the horizon, 

 as shown at a b cdefg of fig. 18 ; but the instant the rod is removed the wing 

 obliterates the angles in question, and flies in an upward and backward direction 

 to its original position as indicated at g h ij k Im of fig. 19. 



Fig. 18. Fig. 19. 



This shows very satisfactorily that while a voluntary effort is required to 

 depress the wing, it is in some measure elevated, and the various inclined 

 surfaces which it makes with the horizon changed by the aid of an elastic 

 ligament or spring common to all wings. The down stroke is readily explained, 

 and its results upon the body obvious. The real difficulty begins with the up or 

 return stroke. If the wing was simply to travel in an upward and backward direc- 

 tion from c to a of fig. 16, page 349, it is evident that it would experience much 

 resistance from the superimposed air, and undo or negative the advantages secured 

 by the descent of the wing. What really happens is this. The wing does not travel 

 upwards and backwards in the direction cb a of fig. 16 (the body be it remembered 

 is advancing), but upwards and forw ards in the direction c d efg. This is brought 

 about in the following manner. The wing is at right angles to the horizon {xx') at c. 

 It is therefore caught by the air because of the more or less horizontal travel of 

 the body at 2, the elastic ligaments and other structures rotating the posterior 

 or thin margin of the pinion in an upward direction, as shown at g h i of figure 

 19, page 351, and d efg of figure 16, page 349. The wing by this partly vital 

 and partly mechanical arrangement is rotated off the wind in such a manner as 

 to keep its dorsal or non-biting surface directed upwards, while its concave or 

 biting surface is directed downwards. The wing, in short, has its planes so 

 arranged, and its angles so adjusted to the speed at which it is travelling, that 

 it darts up a gradient like a true kite, as shown at c d efg of figures 16 and 17, 

 page 349. The wing consequently elevates and propels during its ascent as well 

 as during its descent. It is, in fact, a kite during both the down and up strokes. 

 The ascent of the wing is greatly assisted by the forward travel of the body. 

 It is further assisted by the downward and forward fall of the body. This 

 view will be readily understood by supposing, what is really the case, that 

 the wing is more or less fixed by the air in space at 2 of figure 16, page 349, 

 and that the body, the instant the wing is fixed, falls downwards and forwards 



VOL. XXVI. PART II. 4 Y 



