tage of ftrength and lightnefs ; the webs ar c 
broad on one tide, and more narrow on the 
other; both which contribute to the progrefiive 
motion of the bird, and the clofenefs of the 
wing. Thus each feather takes up a large fur- 
face, but with inconfiderable gravity, fo that 
when the wing is expanded, the animal becomes 
fpecifically lighter than air. The linaller fea- 
thers with which it is cloathed are dffpofed one 
over another in the exa&eft order, lb as to 
lie clofer in proportion to the rapidity of the 
flight. That part of them which is next the 
fkin is furnifhed with a foft and warm down, 
and that next the air with a web on each fide 
of the fhaft, each tingle beard of which is itfclf 
a feather. 
But as this lightnefs of the feathers might 
frequently be impeded by a fhower of rain, or 
any other accidental moiliure, by which means 
the bird might become an eafy prey to every in- 
vader, Nature has provided an expedient where- 
by their feathers are as impenetrable to the wa- 
ter, as by their ftruclure they are to the air. 
Ail birds in general have a receptacle re plenilh- 
ed with oil, fomething in the fhape of a teat, 
and fituated at the extremity of their bodies. 
This teat has feveral orifices, and when the 
bird perceives its feathers to be dry, or expects 
the approach of rain, it fqueezes this teat with 
•the bill, and ftrains from thence a part of the 
