BIRD-FLYING. 
47 
As bird-flying relates to natural winds or the forces of 
the atmofphere, let us for a moment, before going any 
further, conflder the nature and properties of the air. 
<c If we fuppofe the atmofphere, (remarks Dr. 
Lardner,) which extends from the furface of the earth 
upwards to a height more or lefs conflderable, to conflft 
of a feries of layers or ftrata, placed one above the other, 
it is evident that each fucceflive ftratum, in afcending, 
will fuftain a weight lefs than thofe below it. The firft 
ftratum of atmofphere, which is in immediate conta<ft 
with the furface of the earth, is comprefled by the entire 
weight of the atmofphere above it; that is to fay, by the 
weight of the whole atmofphere, except the firft ftratum; 
the next ftratum is comprefled by the weight of the whole 
atmofphere except that of the firft two ftrata; the third 
ftratum is comprefled by the weight of the whole atmof¬ 
phere, except the firft three ftrata; and fo on. Now, it 
has been already fhown that the denfity of air is always pro¬ 
portional to the force which comprefles it; and it follows, 
therefore, that the denfity of the firft, or loweft ftratum, 
is greater than the denfity of the fecond; and the denfity 
of the fecond greater than the denfity of the third ; and 
fo on ; the air becoming gradually lefs denfe as it afcends 
to a greater height.” * * * * * “ The weight of 
mercury is 13$ times greater than that of water, and the 
weight of water is about 800 times that of air, at the mean 
denfity of the latter; confequently, the weight of mer- 
