A
more complete exposition would begin with the rotating earth, and
would superpose the attraction of the moon as a disturbing cause,
treating it as a problem in planetary perturbation, the ocean being
a sort of satellite of the earth. This treatment, introducing
inertia but ignoring friction and land obstruction, gives low water
in the line of pull, and high water at right angles, or where the
pull is zero; in the same sort of way as a pendulum bob is highest
where most force is pulling it down, and lowest where no force is
acting on it. For a clear treatment of the tides as due to the
perturbing forces of sun and moon, see a little book by Mr. T.K.
Abbott of Trinity College, Dublin. (Longman.)

[Illustration: FIG. 113.--Maps showing how comparatively free from land
obstruction the ocean in the Southern Hemisphere is.]

If the moon were the only body that swung the earth round, this is all
that need be said in an elementary treatment; but it is not the only
one. The moon swings the earth round once a month, the sun swings it
round once a year. The circle of swing is bigger, but the speed is so
much slower that the protuberance produced is only one-third of that
caused by the monthly whirl; _i.e._ the simple solar tide in the open
sea, without taking momentum into account, is but a little more than a
foot high, while the simple lunar tide is about three feet. When the two
agree, we get a spring tide of four feet; when they oppose each other,
we get a neap tide of only two feet. They assist each other at full moon
and at new moon. At half-moon they oppose each other. So we have spring
tides regularly once a fortnight, with neap tides in between.

[Illustration: FIG. 114.--Spring and neap tides.]

Fig. 114 gives the customary diagrams to illustrate these simple things.
You see that when the moon and sun act at right angles (_i.e._ at every
half-moon), the high tides of one coincide with the low tides of the
other; and so, as a place is carried round by the earth's rotation, it
always finds either solar or else lunar high water, and only experiences
the difference of their two effects. Whereas, when the sun and moon act
in the same line (as they do at new and full moon), their high and low
tides coincide, and a place feels their effects added together. The tide
then rises extra high and falls extra low.

[Illustration: FIG. 115.--Tidal clock. The position of the di