408 THE EMPIRE OF TITE AIR. 



pelled to reverse liimself, ;i difficult uiaiHeuver, sometimes performed 

 by pigeons in the pigeon house to limber up their Avings. 



To pass from this inclination of some 45° into horizontal course, the 

 bird brings the tail into play. He depresses it, and produces through 

 the jiressure corresponding to the velocity (sometimes aided by a par- 

 ticular light beat) a <lecomposition of forces which results in changing- 

 the course from 45° to the horizontal. Tf the tail is weak, he uses his 

 deltoid mus(!les, which raise his body relatively and so produce the 

 same result. In general, birds often employ both means simultaneously. 



Horizontal motion being attained, the manner of flight is slightly 

 modified witli increasing si)eed. The wing no longer beats j)erpendic- 

 ularly, but is slightly inclined in the direction of the course in order 

 to increase the speed. - 



Soarhu/ JiUjlit. — Some naturalists have advanced most curious ex- 

 planations of bird flight, especially of soaring or sailing flight. For 

 their ])urpose lightness is the main reqiusite. They have pointed to 

 the porous bones, to the spaces filled with air sometimes occurring 

 under the skin of these creatures, as indispensable conditions for sup- 

 port in the air. 



These are fallacies. Birds always exhibit corporal density, practi- 

 cally the same as land animals. DepriAcd of their feathers they sink 

 in water; their specific gravity is about J, as is that of inan, of mam- 

 mals, and of fishes. 



To rightly explain the flight of birds, we must consider them as 

 highly organized machines, which arc sustained through the reactions 

 produced by muscular eftbrt; not as a balloon floating in the air, but 

 as a stone glancing along the water, or a skater gliding over thin ice. 

 All that apparatus described for distending the gannet, the pigeon — 

 all those hollow bones of the pelican, the albatross, etc., serve flight in 

 no degree. Their utility is different. 



After all, expei'iment is <'asy. Strij) a bird of his body feathers 

 leaving him only the wing and feathers, and his flight will be in no way 

 changed; he will get chilled, he will not be able to swim if he is a 

 water bird, but he will assuredly not fly the worse. 



Let us now explain the flight of the soaring 1)irds, or as 1 prefer to 

 call them, the sailers. Birds soar jterfectly in proportion to the mag- 

 nitude of their sustaining surfaces and of the greatness of their mass. 

 This is an unquestionable principle. A large bird, an average sized 

 bird, and a small bird, all thi-ee having the same proportional surface 

 relative to their weight, Avill soar the better the heavier they are. 



Let us consider, then, only the larger birds, for these alone can effect 

 the decomjiositions of wind-force which produce flight without flapping 

 of wing. As the sailing bird first launches out with fiaps, unless from 

 a perch he plunges to get speed, we will suppose him in full action in air, 

 and ])Ossessing initial velocity. He will then glide on rigid wings. If 



