DENSITY of the AIR, is a property that has employ-
ed the later philosophers, since the discovery of the
Torricellian experiment.
It is demonstrated, that in the same vessel, or even
in vessels communicating with each other, at the same
distance from the centre, the air has every where the
same density. The density of air, ceteris paribus, in-
creases in proportion to the compressing powers. Hence
the inferior air is denser than the superior; the density,
however, of the lower air is not proportional to the
weight of the atmosphere on account of heat and cold,
and other causes perhaps which make great alterations
in density and rarity. However, from the elasticity of
the air, its density must be always different at different
heights from the earth's surface; for the lower parts
being pressed by the weight of those above, will be
made to accede nearer to each other, and the more so
as the weight of the incumbent air is greater. Hence
the density of the air is greatest at the earth's surface,
and decreases upwards in geometrical proportion to the
altitudes taken in arithmetical progression.
If the air be rendered denser, the weight of bodies
in it is diminished; if rarer, increased, because bodies
lose a greater part of their weight in denser than in
rarer mediums. Hence, if the density of the air be
sensibly altered, bodies equally heavy in a rarer air, if
their specific gravities be considerably different, will
lose their equilibrium in the denser, and the specifically
heavier body will preponderate. See PNEUMATICS.