the
attention of some of our most advanced scientists, not only in this
country but in other countries also.

Whether the Aether in space is at rest, or is moving along with all the
bodies that float in it, so to speak, is a question of the greatest
importance to scientists and philosophers generally, as the particular
character of the motions of the Aether, which are either suggested or
ascribed to it from the analogies of Nature, are sure to have a most
important bearing not only on the motions of all the planets and
satellites, but also upon such questions as the aberration of light, and
such difficulties as presented by Lord Kelvin in his paper on "Clouds on
the Undulatory Theory of Light" (_Phil. Mag._, July 1902).

I need hardly point out that the hypothesis that Aether is gravitative,
is bound to play a most important part in the consideration and
development of this phase of the study of the universal aetherial
medium. It is not my intention, however, at this stage of the work to go
fully into the development of this aspect of the subject.

The application of this principle will be considered at the right time,
and in the right place. It is, however, generally assumed, that the
Aether is at rest in space, and that the earth, the planets, and the sun
and all stars, move through it with varying velocity, although, as Lord
Kelvin points out, such an assumption is covered with a cloud which up
to the present is "as dense as ever." Of course, if the Aether be at
rest, and the planets and other heavenly bodies move through it with
varying velocity, then the only assumption regarding the Aether is, that
it is frictionless, but, as I have shown in Art. 45, this is opposed to
all philosophical reasoning, and therefore to experience and
observation.

We have, therefore, to postulate for the Aether such motions as shall
fulfil all the Rules of Philosophy, that is, shall be simple in
conception, shall be in harmony with our experience and observation, and
which shall satisfactorily account for the phenomena sought to be
explained, that is, the universal Law of Gravitation; for it is by the
properties, combined with the motions of the Aether, that the physical
cause of Gravitation is alone to be explained.

Let us revert to the question of a stationary Aether for a moment or
two, and let us ask ourselves, where is the evidence for such an
assumption? Has the sun ever ceased to shine, or to send its light-waves
with th