92 
NATURAL "hlSTORY. 
of the wings, ferves it not only as a buoy, 
but alfo as a rudder to direft its flight. 
Plumage . — They are covered with fea- 
thers, moft admirably adapted to the air they 
inhabit, being compofed of a quill, containing 
a confiderable quantity of air, and a fliaft, 
edged on each fide with a moft volatile fub- 
ftance, which, with the concavity of tlie 
wings, renders the body confiderably lighter 
than the air ; and thus enables them to explore 
an immenfc fpace, denied to every other part 
of the creation. 
Sight . — To adapt the fight to the fwiftnefs 
of their motions, their eyes are not fo con- 
vex or prominent as in creatures confined 
to the earth; which not only prevents their 
being injured by the repulfive force of the 
air, in their rapid flights, but likewifc ren- 
ders them lefs liable to be touched with the 
points of thonis, fprays. See. in their pro- 
grefs among trees, bullies, and hedges.-— 
The film, or niftating membrane, with 
which they occalionally cover their eyes, 
without clofing the lids, clears and protefts 
them from the glare of fun-beams, as well 
as from the mills, fogs, and clouds, with 
which the air occafionally abouiKls, when 
forced to range for food or nefting. The 
power alfo of extending the optic nerve, 
gives fuch an acutenefs to their fight, that 
they can perceive objedls more diftindlly, and 
at a greater diftance, than any other crea-. 
ture. 
Hearing- 
