OF NATURAL HISTORY. Si 



progrefs through the air. It is by the inftrumentality of 

 this membrane that the eagle looks at the fun. The feath- 

 ers of all birds are inferted into the fkin in fuch a manner 

 that they naturally lie backward from the head j and allow 

 the rain to run off their bodies, and, by turning their heads 

 in oppofition to the wind, prevent the wind from rumpling 

 their feathers and retarding their flight. Belide this provi- 

 fion, the rump of birds terminates in a large gland, which 

 feeretes an oily fubftance. When the feathers are too dry, 

 or any way difordered, the animals fqueeze this gland with 

 their bills, extracl the oil, and with it they befmear and drefs 

 the feathers. By this means the admillion of water is total- 

 ly prevented. Birds have no feparate ribs ; but the breaft- 

 bone, which Is very large, joins the back-bone, and fupplies 

 their place. 



With regard to the external figure of birds, the form of 

 their bodies is nicely adapted to their manners and the mode 

 of life they are deftlned to purfue. By flriking the air v/ith 

 their wings, they move forward in that element, and their 

 tail ferves them as a rudder to direct their courfe. Their 

 breaft-bone, inftead of being flat, rifes gradually from the 

 fpine and terminates in a fharp ridge or keel, which enables 

 them to cut the air with greater facility. For the fame pur- 

 pofe, the heads of birds are proportionally fmaller than thofe 

 of quadrupeds, and mofl of them terminate in light fharp - 

 pointed beaks. They are likewife deprived of external ears, 

 and of protuberant noftrils. Their tails, inftead of verte- 

 brae, mufcles, and fkin, coniift entirely of featjiers. They 

 have no pendulous fcrotum, no bladder, no flefhy uterus. 

 Neither have they an epiglottis^ though many of them pof- 

 fefs great powers of modulation, and fome of them may 

 even be taught to articulate words. To lighten their beaks, 

 they are deprived of lips and teeth \ and their abdomen or 

 belly ;s proportionally fmall and narrow. 



