68 - NATURAL HISTORY. | 
air’; and thus enables them to explore an immenfe 
{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. convex or promin- 
ent 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 likewife renders 
them lefs liable to be touched with the points of 
thorns, fprays, &c. in ‘their progrefs among trees, 
bufhes, and hedges.—The film, or nictating mem- 
brane, with which they occafionally cover their eyes, 
without clofing the lids, clears and protects them frona 
the glare of fun beams, as well as from the mitts, fogs, 
and ciouds, with which the air occafionally abounds, 
when forced to range for food or nefting. “The pow- 
er alfo of extending the optic nerve, gives fuch an a- 
cutenefs to their fight, that they can perceive objecis : 
more diftinctly, and ata greater diftance, than any 
other creature. | ae 

founds, without any external ear, which would n 
only impede their flight, but render them liable t 
-maiy injuries in darting through bufhes, briars, &c. 
HEARING..... Phey have the power mich world na 
SMELLING..... Lheir {cent is fo very acute and ex- 
tenfive,. by which they are apprifed of the approach 
“of their natural, as well as artificial enemies, that 
thofe who decoy ducks, are obliged to keepa piece of 
burning turf in their-mouths, to prevent being dif- 
covered. | 
INTERNAL STRUCTURE...,.1. he bones, which are 
formed fufficiently {trong to fupport the weight of the 
body, and the fy{ftem of its functions, are fo light, as 
to be fcarcely any additional burthen to the flefh. All 
their internal ftructure is calculated to increafe the 
furface beyond the proportion of the folidity of their — 
bodies, in order to render them lighter than the fame — 
portion of air. The lungs and ends of the windpipe 
branches imbibe air into a number of bladder recept- 
acles. The crop, which is the repofitory for fuper- — 
fluous toad ,signblirs them in long flights, and other 
oe mt oak 
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