AERONAUTICS. 



219 



forming, and to avoid natural currents when not adapted for 

 propelling or sustaining purposes (fig. 67, 68, 69, and 70, 

 p. 141). 



If any one watches an insect, a bat, or a bird when dressing 

 its wings, he will observe that it can incline the under sur- 

 face of the wing at a great variety of angles to the horizon. 

 This it does by causing the posterior or thin margin of the 

 wing to rotate around the anterior or thick margin as an 

 axis. As a result of this movement, the two margins are 

 forced into double and opposite curves, and the wing con- 

 verted into a ^plastic helix or screw. He will further observe 

 that the bat and bird, and some insects, have, in addition, the 

 power of folding and drawing the wing towards the body 

 during the up stroke, and of pushing it away from the body 

 and extending it during the down stroke, so as alternately to 

 diminish and increase its area; arrangements necessary to 

 decrease the amount of resistance experienced by the wing 

 during its ascent, and increase it during its descent. It is 

 scarcely requisite to add, that in the aeroplanes and aerial 

 screws, as at present constructed, no provision whatever is 

 made for suddenly increasing or diminishing the flying sur- 

 face, of conferring elasticity upon it, or of giving to it that 

 infinite variety of angles which would enable it to seize 

 and disentangle itself from the air with the necessary 

 rapidity. Many investigators are of opinion that flight is 

 a mere question of levity and power, and that if a machine 

 could only be made light enough and powerful enough, 

 it must of necessity fly, whatever the nature of its flying 

 surfaces. A grave fallacy lurks here. Birds are not more 

 powerful than quadrupeds of equal size, and Stringfellow's 

 machine, which, as we have seen, only weighed 12 lbs., 

 exerted one-third of a horse power. The probabilities there- 

 fore are, that flight is dependent to a great extent on the 

 nature of the flying surfaces, and the mode of applying those 

 surfaces to the air. 



Artificial Wings (Borelli's Views). — With regard to the 

 production of flight by the flapping of wings, much may and 

 has been said. Of all the methods yet proposed, it is unques- 

 tionably by far the most ancient. Discrediting as apocryphal 

 the famous story of Daedalus and his waxen wings, we cer- 



