me, so that
while each ray has for its path a straight line, yet those rays are
emitted equally on every side. In like manner, though the direction of
the forces between two attracting bodies is that of a straight line, yet
the law of universal attraction is equally exerted on every side of the
planet at one and the same time.

In the theory of the Aether, therefore, to be developed in this work, it
will have to be demonstrated that the direction of the forces, which are
originated and transmitted by that physical medium, must philosophically
fulfil the conditions which govern the direction of the forces, as
observed in gravitational phenomena.

ART. 21. _Proportion of the Forces._--Newton proved that the attraction
is proportional to the product of the masses of the bodies concerned.

Hence it is that the sun, which is the centre of the solar system, is
capable of attracting the most remote planets, because the mass of the
sun is greater than the mass of all the planets put together. Or take
another illustration. Suppose that the sun and the earth are at equal
distances from Saturn. Now the sun's mass is about 300,000 times that of
our earth. Therefore if the earth draws Saturn through a certain
distance in one second, the sun would draw Saturn through a distance
which is 300,000 greater than the earth in the same period.

The governing principle, therefore, which decides the proportion of the
attractive forces between two bodies is mass, and not simply density or
volume. The mass of a body is a property which remains the same, as long
as the inertia of the body remains constant. Mass is really a measure of
the inertia of a body, or that property of a body by which it continues
in its state of motion or of rest.

Mass is therefore a compound quantity, being equal to volume multiplied
by density, so that if the volume of any body is halved, the density is
doubled. Thus, the proportion of the attractive force between any two
bodies ever remains the same, so long as the masses of the two bodies
remain the same. Through all the changes of volume and density of any
body, its attractive force remains constant, as long as the mass remains
constant; for the simple reason, that as the volume of a body is
increased, the density is proportionately decreased; or, as the volume
is decreased, the density is increased.

For example, the volume of the sun as compared with the volume of the
earth, is about 1,300,000 times great