tability and proof that perhaps it had never received before.

Further evidence of the existence of the universality of the attractive
force, is to be found in a certain system of stars known as binary
stars, which revolve around each other, while they gravitate around a
common centre. Recent researches in astronomy only seek more and more to
confirm the universality and effectiveness of this grand law, that seems
to hold the entire universe in its sway.

Any medium, therefore, which is postulated as the physical cause of
Gravitation, must itself be as universal as Gravitation, in order for it
to be able to fulfil this condition of universality. We shall find, as
we proceed, that the only possible medium which can fulfil this
condition, is the universal Aether, whose qualities and properties are
already partly known and partly understood.

ART. 20. _Direction of the Forces._--The attraction of Gravitation is
always directed along the straight line which joins the centres of
masses of the attracting and attracted bodies. Thus, if the earth and
moon are taken as examples, an imaginary straight line drawn from the
centre of the earth's mass to the centre of the mass of the moon would
be the direction in which the gravitative force would be exerted. Now a
line which joins the central body to its satellite we shall see when we
come to deal with Kepler's Laws is known as the Radius Vector. Thus the
path of the attraction between the two bodies is along the Radius
Vector. It is a singular coincidence that the path of a ray of light
from the sun also coincides with the Radius Vector, as it is one of the
laws of light that the path of a ray always follows a straight line.

It must not, however, be assumed, that while the attractive power is
being exerted along any one straight line joining the centres of two
bodies, therefore the attractive power is not operative in relation to
any other part of the space, around the body. If our earth, for example,
had four moons instead of one, and they were each in different positions
in relation to the earth, then the law as to the direction of the forces
would still hold good. We have examples of this in the case of Jupiter
with his five moons, and Saturn with his eight moons. So that the
attractive force of Gravitation is again like light, it operates on all
sides equally at one and the same time. A lamp in the middle of a room
sends its light waves on every side at one and the same ti