ow tides here are
therefore at 6 a.m. and 6 p.m., and the 6 p.m. low tide is a nuisance to
the river steamers. The spring tides at London are highest about
half-past two.

* * * * *

It is, therefore, quite clear that the moon has to do with the tides. It
and the sun together are, in fact, the whole cause of them; and the mode
in which these bodies act by gravitative attraction was first made out
and explained in remarkably full detail by Sir Isaac Newton. You will
find his account of the tides in the second and third books of the
_Principia_; and though the theory does not occupy more than a few pages
of that immortal work, he succeeds not only in explaining the local
tidal peculiarities, much as I have done to-night, but also in
calculating the approximate height of mid-ocean solar tide; and from the
observed lunar tide he shows how to determine the then quite unknown
mass of the moon. This was a quite extraordinary achievement, the
difficulty of which it is not easy for a person unused to similar
discussions fully to appreciate. It is, indeed, but a small part of what
Newton accomplished, but by itself it is sufficient to confer
immortality upon any ordinary philosopher, and to place him in a front
rank.

[Illustration: FIG. 110.--Whirling earth model.]

To make intelligible Newton's theory of the tides, I must not attempt to
go into too great detail. I will consider only the salient points.
First, you know that every mass of matter attracts every other piece of
matter; second, that the moon revolves round the earth, or rather that
the earth and moon revolve round their common centre of gravity once a
month; third, that the earth spins on its own axis once a day; fourth,
that when a thing is whirled round, it tends to fly out from the centre
and requires a force to hold it in. These are the principles involved.
You can whirl a bucket full of water vertically round without spilling
it. Make an elastic globe rotate, and it bulges out into an oblate or
orange shape; as illustrated by the model shown in Fig. 110. This is
exactly what the earth does, and Newton calculated the bulging of it as
fourteen miles all round the equator. Make an elastic globe revolve
round a fixed centre outside itself, and it gets pulled into a prolate
or lemon shape; the simplest illustrative experiment is to attach a
string to an elastic bag or football full of water, and whirl it round
and round. Its prolateness is re