But it was part of the merit of Newton's mighty theory of gravitation
that it furnished an explanation even of this age-old mystery. We can
see, in broad outlines at any rate, that the theory of universal
attraction can be applied to this case. For the moon, Newton taught us,
pulls every particle of matter throughout the earth. If we imagine that
part of the earth's surface which comprises the Pacific Ocean, for
instance, to be turned towards the moon, we see that the moon's pull,
_acting on the loose and mobile water_, would tend to heap it up into a
sort of mound. The whole earth is pulled by the moon, but the water
is more free to obey this pull than is the solid earth, although small
tides are also caused in the earth's solid crust. It can be shown also
that a corresponding hump would tend to be produced on the other side of
the earth, owing, in this case, to the tendency of the water, being more
loosely connected, to lag behind the solid earth. If the earth's surface
were entirely fluid the rotation of the earth would give the impression
that these two humps were continually travelling round the world, once
every day. At any given part of the earth's surface, therefore, there
would be two humps daily, i.e. two periods of high water. Such is the
simplest possible outline of the gravitational theory of the tides.
[Illustration: THE CAUSE OF TIDES
The tides of the sea are due to the pull of the moon, and, in lesser
degree, of the sun. The whole earth is pulled by the moon, but the loose
and mobile water is more free to obey this pull than is the solid earth,
although small tides are also caused in the earth's solid crust. The
effect which the tides have on slowing down the rotation of the earth is
explained in the text.]
[Illustration: _Photo: G. Brocklehurst._