adily visible.

Now consider the earth and moon revolving round each other like a man
whirling a child round. The child travels furthest, but the man cannot
merely rotate, he leans back and thus also describes a small circle: so
does the earth; it revolves round the common centre of gravity of earth
and moon (_cf._ p. 212). This is a vital point in the comprehension of
the tides: the earth's centre is not at rest, but is being whirled round
by the moon, in a circle about 1/80 as big as the circle which the moon
describes, because the earth weighs eighty times as much as the moon.
The effect of the revolution is to make both bodies slightly protrude in
the direction of the line joining them; they become slightly "prolate"
as it is called--that is, lemon-shaped. Illustrating still by the man
and child, the child's legs fly outwards so that he is elongated in the
direction of a radius; the man's coat-tails fly out too, so that he too
is similarly though less elongated. These elongations or protuberances
constitute the tides.

[Illustration: FIG. 111.--Earth and moon model, illustrating the
production of statical or "equilibrium" tides when the whole is whirled
about the point G.]

Fig. 111 shows a model to illustrate the mechanism. A couple of
cardboard disks (to represent globes of course), one four times the
diameter of the other, and each loaded so as to have about the correct
earth-moon ratio of weights, are fixed at either end of a long stick,
and they balance about a certain point, which is their common centre of
gravity. For convenience this point is taken a trifle too far out from
the centre of the earth--that is, just beyond its surface. Through the
balancing point G a bradawl is stuck, and on that as pivot the whole
readily revolves. Now, behind the circular disks, you see, are four
pieces of card of appropriate shape, which are able to slide out under
proper forces. They are shown dotted in the figure, and are lettered A,
B, C, D. The inner pair, B and C, are attached to each other by a bit of
string, which has to typify the attraction of gravitation; the outer
pair, A and D, are not attached to anything, but have a certain amount
of play against friction in slots parallel to the length of the stick.
The moon-disk is also slotted, so a small amount of motion is possible
to it along the stick or bar. These things being so arranged, and the
protuberant pieces of card being all pushed home, so that they are
hidd