as yet been compressed only into
one-eighth of its original bulk.

For every degree of heat between the freezing-point and the boiling-point,
32 deg. and 212 deg., the expansion of air is about 1/490th part, so that
any invention which seeks to use rarefied air as a motive power
must employ a very intense degree of heat, enough to fuse many kinds
of metals.

To the celebrated Mr. Boyle and to Henry Cavendish, both of Great
Britain, we are indebted for most of what we know of this particular
property of the air.

* * * * *

Density, or closeness, is another quality of the atmosphere. It has
been found to be 770 times less than that of water, and 770 cubic
inches of air weigh as much as a cubic inch of water. It is in direct
ratio with its elasticity, and there are tables by which it may be
determined at different altitudes. At the surface of the earth, this
density is indicated as 1; at 2-1/2 miles, as 1/2; at 5 miles, as 1/4;
and so on, the difference being in a geometrical progression.

As we proceed in the consideration of our general subject, we shall
find, under the appropriate heads, that density is not without
material influence on reflection and refraction, on transparency and
the transmission of light, the presence or absence of moisture, and
the amount of heat at the earth's surface,--and we might add, on
health, and the increase or diminution of the vital energies.

* * * * *

Temperature is another branch of our subject, and one involving a
series of subordinate topics on which volumes have been written, and
to which are still devoted the labors of the most learned men of our
day. In this place, merely an out-line can be attempted.

Temperature is the degree of heat or cold in the particles of all
bodies, which is perceptible by sensation, and is measurable by their
expansion or contraction. It is the key to the theory of the winds, of
rain, of aerial and oceanic currents, of vegetation and climate with
all their multifarious and important differences. While the inclined
position of the earth on its axis and its movement in its elliptical
orbit influence the general amount of heat, it is rather to the
consequences of these in detail that we are called when we speak of
temperature. If the sun shone on a uniformly level surface, everywhere
of the same conducting and radiating power, there would be but little
difficulty in tracing the monot