228 



AERONAUTICS. 



If we take the resultant of these two forces, we have at most 

 propulsion in the direction a h. This, moreover, would only 

 hold true if the bird was as light as air. As, however, gravity 

 tends to pull the bird downwards as it advances, the real 

 flight of the bird, according to this theory, would fall in 

 a line between h and /, probably in xh. It could not possibly 

 be otherwise ; the wing described and figured by Borelli and 

 Marey is in one piece, and made to vibrate vertically on either 

 side of a 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 resist- 

 ance during the up stroke^ when it is acting against gravity ^ 

 than during the down stroke^ when it is acting with gravity. 



Fig. 114. Fig. 115. 



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 Borelli 

 and Marey is not a flying wing, but a mere propelling ap- 

 paratus, seems evident to the- latter, for he states that the 

 winged machine designed by him has unquestionably not 

 motor power enough to support its own weight} 



The manner in which the natural wing (and the artificial 

 wing properly constructed and propelled) evades the resistance 

 of the air during the up stroke, and gives continuous support 

 and propulsion, is very remarkable. Fig. 115 illustrates the 

 true principle. Let a h represent the horizon ; m n the direc- 

 tion of vibration; xs the wing ready to make the down 

 stroke, and xt the wing ready to make the up stroke. When 

 the wing xs descends, the posterior margin {s) is screwed 



1 Revue des Conrs Scieiitifiques de la France et de TEtranger. 8vo. March 

 20, 1869. 



