THE EMPIKE OF THE All?. 



455 



When a t>ii(l is swecpin.u in a circle, the w iii:^' iM)iiitiii.i;- towa.ids the 

 center of that circle is always less extended Mian tiiat wliich sweeps 

 the circnnitereiice, so that when a sailin,u' bird is seen slightly to fold 

 one of liis wings, it may l)e known that lie is about to turn towards 

 that side. 



Fl(i. 14.— Wiiiiis ivtr:irte(l 



The whole l)ody aids in this movement: the whole bird bears itself 

 to that side, the tail, even when rudimentary and heiu-e feeble in action, 

 concurs in the execution of tliis maiuenver. It is an instinctive action 

 among the feather<'d tribe, Just as with man. wlu'ii he uses his arms to 

 equilibrate himself on his legs. 



The two wings never balance each other perfectly; one side is 

 always heavier than the other, as the surfaces are not equally divided. 

 Differences in weights and surfaces cause a tilt towards the side most 

 lieavily loaded or exj)osing least surface, as the case maybe; hence 

 aeroplanes, wliether machines or birds, always tend to sweep to one 

 side or the other. To obtain rectilinear [)rogTession some corrective 

 force must intervene. In the animated ai-roplane this force consists in 

 life: in artificial flying apparatus man will needs i)rodiu;e this force. It 

 might be i)0ssible to jn'oduce rectilinear progression automatically, in 

 large aero]iIaues, by means of electrical apparatus, in which contacts 

 wouhl be made through the use of nu'rcury which would seek its level. 



When we observe attentively a sailing bird gliding in a strong and 

 irregular current of wind, we are struck with the lapidity with which 

 the center of gravity is shifte<l in order to satisfy the needs for supi)()rt 

 and for maintaining the course. A ])u1f of wind immediately results 

 in a Hexing of wings, their tips swing to the rear, the center of grax- 

 ity advances in consequence, and thus neulrali/.es the increased i)res- 

 sure ]>roduced by tin* acceleration in the current of air. 



This adroit evolution, performed just at the right time, which at first 

 sight seems to be an instincti"\(' action of the creature, is probably after 

 all simply automatic. We may be assured that this change in form of 

 surface, this alteration in the ecpiipoise is not jn-oduced by tlie con- 

 scious vital acti(Ui of the ner\'«'s, but is simply a phenomenon of nniscu- 

 lar elasticity. The bird receives the shock of the wind unconsciously, 

 his attention is otherwise engaged, his wings ar<' stretched at their 



