[ 212 1 
without adding to the weight of the whole ; which 
will rather indeed be diminifhed by the difference 
of fpecific gravity, between the external and in- 
ternal air. This opinion was ftrengthened, by ob- 
ferving, that the feathers of birds alfo contain a 
confiderable quantity of air, and in the very part 
which requires the greateft flrength ; as likewife by 
the analogy of fifh, which have air contained in 
their bodies, to leffen their fpecific gravity 3 though 
the ufe of this, in fifh, which are to move in a 
much heavier element, is more obvious than in 
bird's. 
But when I found the ostrich, which is not in- 
tended to fly, endowed with nearly the fame con- 
ftruCtion and that the common fowl, and many 
others of that clafs, which do fly j and alfo the 
wood-cock, which flies, and is luppofed to be a 
bird of paffage, are not fo well provided with air, as 
the ostrich ; and that the eatt, which flies, dif- 
fers not, in that way, from animals that do not 
fly ; fo many contradi&icns to the theory, obliged 
me to think of fome other ufe, for this lingular 
mechanifm. 
The next conjecture, that offered itfelf, was, 
that thefe parts. were to be confidered as an 
appendage to the lungs. The analogy of amphi- 
bious animals leads to this j for, in many of them, 
the fnake, viper, &c. the lungs are continued down 
through the whole belly, in form of two bags, 
the upper part only of which can do the office of 
refpiration ; and therefore, the remainder is to be 
confidered as a refervoir of air. Now there is a 
great fimifarity between birds and that clafs of ani- 
mals, 
