INTRODUCTION. 
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Inftead of the large head and formidable jaws, the deep capa- 
cious ched, the brawny (houlders, and linewy legs of the qua- 
drupeds, we obferve the pointed beak, the long and pliant neck, 
the gently fwelling (boulder, the expanfive wings, the tapering 
tail, the light and bony feet ; all which are wifely calculated to 
affjift and accelerate their motion through the yielding air. Eve- 
yy part of their frame is formed for lightnefs and buoyancy ; 
their bodies are covered with a foft and delicate plumage, fo dif- 
pofed as to protedf them from the intenfe cold of the atmofphere 
through which they pafs 5 their wings are made of the lighted 
materials, and yet the force with which they drike the air with 
them is fo great as to impel their bodies forward with adonifh- 
ing rapidity, whild the tail ferves the purpofe of a rudder to di- 
rect them to the different objects of their purfuit. The internal 
ftru&tire of birds is no lefs nicely adapted to the fame wife 
and ufeful purpofes ; all their bones are light and thin, and all 
the mufcles, except thofe which are appropriated to the purpofe 
of moving the wings, are extremely delicate and light ; the 
lungs are placed clofe to the back-bone and ribs, the air en- 
tering into them by a communication from the windpipe, pad 
fes through and is conveyed into a number of membranous cells 
which lie upon the Tides of the pericardium, and communicate 
with thofe of the dernum. In fome birds thefe cells are con- 
tinued down the wings, and extend even to the pinions, thigh 
bones, and other parts of the body, which can be filled and did 
tended with air at the pleafure of the animal. 
The extreme Angularity of this almod univerfal diffufion of 
air through the bodies of birds naturally excited a drong defire 
to difcover the intention of Nature in producing a conformation 
fo extraordinary. The ingenious Mr Hunter imagined that it 
might be intended to afiid the animal in the a& of flying, by 
increafing its bulk and drength without adding to its weight. 
This opinion was corroborated by confidering, that the feathers 
of birds, and particularly thofe of the wings, contain a great 
quantity of air. In oppofition to this he informs us, that the 
Qftrich, which does not fly, is neverthelefs provided with air 
