re in the relation of
299 to 300.

Astronomers have ascertained the deflection of plumb lines from the
vertical, by the action of mountains. The attraction of a projecting
mass of known bulk and density, with one whose bulk alone is known, is
thus determined, and hence the density of the latter may be calculated.

Even comparatively small masses of matter may be placed under such
circumstances at the surface of the earth, that their mutual action can
be observed uninfluenced by the preponderating attraction of the earth,
and thus a new means of comparison obtained.

The pendulum whose vibrations ought to vary according to a definite law,
as we recede from the surface of the earth, has that law affected by the
elevated ground on which it is placed, and here again a comparison may
be instituted between the general and local attractions.

All these modes of investigation concur in, and confirm the general
result, that the mean density of the earth is about five times as great
as that of water. Now as a great portion of the surface is composed of
that fluid, and as the general density of the land is little more than
twice as great as that of water, it follows incontestably that the
interior of the earth is far more dense than its outer covering.

All material substances are capable of assuming, under proper
modifications of latent heat, either the solid, the liquid, or the
gaseous form; yet all are beyond doubt composed of atoms, solid, hard,
and incapable of further division. Under their own mutual attraction
these particles tend to unite, and cohere in solid masses, and to this
attractive force the repulsive power of heat is constantly opposed,
tending to prevent their aggregation, and retaining them, according to
its intensity, in the gaseous or liquid form.

The heat necessary to maintain these states of existence in bodies, may
be produced in various ways. Our usual experience leads us to consider
it as more generally arising from two causes, radiation from the sun,
and the chemical action causing combustion. The former could never have
produced the temperature known to exist at present upon the surface of
the globe, for the earth radiates as well as the sun, and is constantly
throwing off heat into the surrounding space. We know that these two
actions have for twenty centuries exactly balanced each other, and that
the mean temperature of the earth has neither increased nor diminished
in all that period. Had the