L54 



THE EMPIRE OF THE AIR. 



THEOEY OF THE ABKOPLANE. 



Vertical and horizontal equilihriirm. — To clianjie the equipoise of bis 

 aeroplane in the vertical direction, the sailing bird makes use of bis 

 tail, which under the action of the imping air serves in all respects as 

 a rudder; but be has a much more energetic means of displaying bis 

 center of gravity, which consists in altering bis center of tignre; thiit 

 is to say by changing the form of bis sustaining surface, and by dis- 

 placing it in relation to his body. 



Fig. 12. — Winus in iionunl attitudr 



When the bird has disposed bis organs of sailing iligbt in proper 

 equipoise, when bis ai-roijlane is set for efficient progress, as for example 

 in Fig. 12, should any necessity whatever require him to ascend sud- 

 denly, he will not enii)l()y bis tail for that purpose, especially if it be a 

 feeble one, because it will not produce snfficient action, but be stretches 

 Ills wings forward, Fig. 1.). His center of gravity and bis center of 

 hgure thus recede decidedly to the rear, and upward gliding and ascen- 

 sion must follow. 



Fk;. It!. — AA'ings jn-ojcctetl. 



If, on the other hand, the bird assumes the following attitude. Fig. 14, 

 the center of gravity being carried forward produces downward gliding. 



These displacements, jn^oduced at will by the variable position of tiie 

 wings and the guidance obtained by the action of the air on the tail, 

 constitute the bird's directing })ower in a vertical direction. 



As to gnidance in the horizontal direction, it is very simply brought 

 about. It is also almost always procured through the derangement in 

 the equipoise of the a-'roplane, except with birds having very ample 

 tails and tlius possessing an organ capable of •this service; these use 

 it constantlv, as witness the naucler and the kite. 



