but it would be a grave error to think that ether
is the only possible basis for such unity, or to make it an essential
part of one's philosophy of the universe. Ether was never more than an
imagined entity to which we ascribed the most extraordinary properties,
and which seemed then to promise considerable aid. It was conceived as
an elastic solid of very great density, stretching from end to end of
the universe, transmitting waves from star to star at the rate of
186,000 miles a second; yet it was believed that the most solid matter
passed through it as if it did not exist.

Some years ago a delicate experiment was tried for the purpose of
detecting the ether. Since the earth, in travelling round the sun, must
move through the ether if the ether exists, there ought to be a stream
of ether flowing through every laboratory; just as the motion of a ship
through a still atmosphere will make "a wind." In 1887 Michelson and
Morley tried to detect this. Theoretically, a ray of light in the
direction of the stream ought to travel at a different rate from a ray
of light against the stream or across it. They found no difference, and
scores of other experiments have failed. This does not prove that there
is no ether, as there is reason to suppose that our instruments would
appear to shrink in precisely the same proportion as the alteration of
the light; but the fact remains that we have no proof of the existence
of ether. J. H. Jeans says that "nature acts as if no such thing
existed." Even the phenomena of light and magnetism, he says, do not
imply ether; and he thinks that the hypothesis may be abandoned. The
primary reason, of course, for giving up the notion of the ether is
that, as Einstein has shown, there is no way of detecting its existence.
If there is an ether, then, since the earth is moving through it, there
should be some way of detecting this motion. The experiment has been
tried, as we have said, but, although the method used was very
sensitive, no motion was discovered. It is Einstein who, by
revolutionising our conceptions of space and time, showed that no such
motion ever could be discovered, whatever means were employed, and that
the usual notion of the ether must be abandoned. We shall explain this
theory more fully in a later section.

INFLUENCE OF THE TIDES: ORIGIN OF THE MOON: THE EARTH SLOWING DOWN

Sec. 16

Until comparatively recent times, until, in fact, the full dawn of
modern science, the tides