DR PETTTGREW ON THE PHYSIOLOGY OF WINGS. 415 



port) ; when the wing x d ascends it will endeavour to rise in the direction g, 

 as it darts up like a kite (the body bearing it being in motion). If we take the 

 resultant of these two forces, we have at most propulsion in the direction a b. 

 This, moreover, would only hold true if t/ 



the bird was as light as air. As, how- X. 



ever, gravity tends to pull the bird down- > ' 



wards as it advances, the real flight of / \. 



the bird, according to this explanation, J ^ _ ; 



would fall in a line between b and /, \^"^/ 



probably in w h. It could not possibly j *- ■' W 

 be otherwise ; the wing described and / 



figured by Marey is in one piece, and J 



vibrated vertically on either side of a g ' 55 ' 



given line. If, however, a wing in one piece is elevated and depressed in a 

 strictly perpendicular direction, it is evident that the wing will experience a 

 greater resistance during the up sfroke, when it is acting against gravity, than 

 during the down stroke, when it is acting with gravity. As a consequence, the 

 bird will be more vigorously depressed during the ascent of the wing than it 

 will be elevated during its descent. That the mechanical wing referred to 

 by Marey is not a flying iving, but a mere propelling apparatus, seems evident 

 to himself, for he states that " the winged machine designed by him has un- 

 questionably not motor power enough to support its oivn weight."* 



The manner in which the natural wing (and the artificial wing properly con- 

 structed and propelled) evades the resistance of the air during the up stroke, 

 and gives continuous support and propulsion, is very remarkable. Fig. 56, page 

 416, will illustrate the principle. Let a b represent the horizon ; m n the direction 

 of vibration ; x s the wing ready to make the down stroke, and x t the wing ready 

 to make the up stroke. When the wing x s descends, the posterior margin (s) 

 is screwed doivnivards and forwards in the direction s, t, the forward angle 

 which it makes with the horizon increasing as the wing is lowered. The air is 

 thus seized by a great variety of inclined surfaces, and as the under surface of 

 the wing, which is a true kite, looks upwards waft, forwards, it tends to carry the 

 body of the bird upwards and forwards in the direction x iv. When the wing 

 x, t makes the up stroke, it rotates from below upwards to prepare for the second 

 down stroke. The wing does not, however, ascend in the direction t, s. On the 

 contrary, it darts up like a true kite, which it is, in the direction x, v, in virtue of 

 the reaction of the air, and because the body of the bird, to which it is attached, 

 had a forward motion communicated to it by the wing during the down stroke. 

 The resultant of the forces acting in the lines x v and x b, is one acting in the 



* Revue des Cours Scientifiques de la France et de rEtranger. 8vo. March 20, 18G9. 

 VOL. XXVI. PART II. 5 P 



