B I R 
ftill; and fiflies yet more; but infefts afford fo 
very great a variety, that they elude the refearches 
of the moft inquifitive naturalifts. Quadrupeds 
are obferved to bear fome diftant refemblance, in 
their internal ftru6lure, toman; but that of Birds 
is entirely diflimilar: formed chiefly to inhabit the 
empty regions of air, all their parts are fuited to 
that purpofe; the flaape of their bodies^ which is 
iharp before, in order to facilitate their paflage 
through the yielding element, rifes by a gradual 
fwell, and falls off in expanfive tails, which ierve 
to keep them buoyant while their fore-parts are 
cleaving the air. They have been, for this reafon, 
not unaptly compared to a veffel making it's way 
through the water; the trunk of the body of the 
animal anfwering to the hold, the head to the prow, 
the tail to the ruddec, and the wings to the oars. 
The external formation of Birds affords matter 
for our admiration: particularly the pofition of their 
feathers, which generally tend backwards; and 
thus, by lying one way, and over each other in an 
exacl and regular order, anfwer the threefold pur- 
pofe of warmth, fpeed, and fecurity. Thofe parts 
of the feathers next their bodies are furnifhed v/ith 
a' warm and foft down ; while the external ones are 
arrayed with double beards in two ranks, longer at 
one end thari at the other: thefe beards confift of 
rows of little thin laminae, difpofed andinferted in 
lines, as perfe6l and regular as if their extremities 
had been cut off by the moft curious artift. But, 
left thefe feathers fliould receive any injury from 
their violent attrition againft the air, or imbibe the 
moifture of the atmofphere. Birds are furniflied 
v/ith glands behind, containing proper quantities 
of oil, which they occafionally prels out with their 
bills, and fpread over all thofe feathers which re- 
quire fmoothing: thefe glands, which are fituated 
on their rumps, are furnilhed with apertures, round 
which grow fmall tufts of feathers fomewhat like 
painters pencils. Such poultry, however, as live 
principally under cover, and feldom expand tlieir 
wings in flight, have a fmaller quantity of this fluid 
than thofe which frequent the open air; the fea- 
thers of a hen, for inftance, are pervious to every 
fhower; but thofe of a fwan, a goofe, aduck, or 
a moor-hen„ as well as all llich Birds as nature has 
direfted to an aquatic life, are dreffed with oil 
from the very firft day of their leaving their fhells ; 
and thus: their ftocks of fluid are proportioned to 
the neceffary degrees of confumption. Their flefh, 
indeed, contrails a flavour from it, which in fome 
is fo very rancid as to be totally unfit for food; 
but if their flefliis injured by it, their feathers are 
improved, and made more valuable for all the 
domeftic purpofes to which they are ufually ap- 
plied. 
The feathers with which Birds are cloathed are 
equally worthy of admiration : the fhaft of each is 
formed proportionably ftrong, but hollow below, 
in order to contribute to it's lightnefs ; and is filled 
above with pith, to afford nourifhment to the beard 
fpringingfrom the fhaft of the feather on either fide. 
Thefe feathers, with refpedl to their lengths and 
ftrength, are generally fo placed, as that the largeft 
and ftrongeft may have the greateft fliare of duty 
in flight. Nor is the vane, or beard, of the fea- 
ther, formed with lefs wifdom or art : it does not 
confift of one continued membrane, becaufe, if that 
were broken, it could not eafily be repaired; but is 
compofed of many layers, fornewhat refembling 
feathers, and lying againft each other in clofe con- 
jundibn. Towards the fliaft of the feather,, thefe 
layers are broad, and of a femicircular form, f6r tar 
purpofe of adding to their ftrength, as well as to 
keep them clofer to each other when in aft'on. 
They, however, grow flender and taper toward the 
exterior edge of the vane, for the purpofe of con.^ 
tributing to their lightnefs. On their inferior fides, 
they are thin and fmooth ; but tJieir upper ex- 
ternal edges are parted into two hairy margins, hav - 
ing a different fort of hair on eacji fide, broad at 
the bottom, and fiendcr and bearded above. By 
this miechanifra, the hooked beards of one layej? 
alv/ays lie near the ftraight beards of the fuccceding 
one, and by that means lock and hold each odier- 
The wings of thofe Birds which fly are ufually 
placed at thofe parts of their bodies wiiicli ferve t^a 
poife and fupport them in the air : they anfv/er t«> 
the fore-legs in quadrupeds; and at their extre- 
mities they have a finger like an appendix,, ufually 
called the baftard-wing. This auxiliaiy infti-ument 
of flight is likewife furniflied with quills, whicfi 
differ from the commiOn feathers only in the large- 
n.efs of their fize, and their fpringing from the deep- 
er part of the fkin, their fhafis lying almoft cioj« 
to the bone : the beards of thefe quiiis are broad 
on one fide, but more narrow on the other; and 
both contribute to the progreffive motion of the 
Birds, and the clofenefs of their wings. 
All Birds are furniflied with tv/o very ftrong 
perioral mufcles on each fide of their breaft-bones. 
In quadrupeds, as well as in men, the peroral 
mufcles are. trifling in comparifon with thofe of 
Birds. In the former, the mufcles of the thighsj, 
and the hinder-parts of the body,, are by far th*j 
ftrongeft : but, in Birds, it is far otherwife; the pec- 
toral mufcles which give motion to their wings, o? 
arms, are amazingly ftrong; while thofe of theii; 
thighs are weak and flender. By means of thefe, 
a Bird can move it's wings witLa degree of ftrength 
which is almoft incredible: the flap of a fwan's 
v/ing would break the leg of a man ; and a fimiiar 
blow from an eagle has been known tooccafion in- 
ftant death. Such, confequently,, is the force of the 
wing, and fuch it's lightnefs, as to be inimitable by 
human art : and as no machines v/hich human fkili: 
can contrive are capable of imparting fo great 
power to fo light an apparatus^ the art of flying 
v/ith artificial wings, fo often and fo fruitlefsly 
fought after, appears to be totally unattainable. 
In all Birds, no6turnal ones excepted, their heads ^ 
in proportion to their bodies, are fmaller than thofe 
of quadrupeds, that they may with greater fticility 
divide the air in the a6t of flying, as well as prepare 
a more eafy paffage for their bodies. Their eyes 
alfo are more flat and depreffed than thofe of 
quadnipeds ; and their pupils are encompaffed bv 
fmall plates of bone under the exterior coats of the 
organs of fight, in order to ftrengthen and defend 
them from injuries. Birds have alfo a kind of ftcins^ 
called the ni6litating membranes, with which they 
can cover their eyes at pleafure, as with a veil^ 
though their eye-lids continue open r thefe mem- 
branes orio-inate from the greater or miore obtufe 
angles of their eyes ; and probably ferve to wip$;_ 
cleanfe, and moiften their ftirfaces. 
The eyes of Birds are admirably adapted to vi- 
fion, by a particular expanfion of their optic nerve.;?^ 
which renders the impreffion of external objetfts 
more vivid and diftiadl. From this peculiar con- 
formation, it appears that the faculty of fight in 
Birds is infinitely fuperior to that of other animals i 
and, indeed, is indifpenfably neceffary to their fup- 
port and fecuritv. Were the eye lefs perfeft, tlae 
■ - ' . Bird^ 
