liquid portions of the earth. The waters which
are turned towards the moon, being nearer to it
than the mass of the terrestrial globe, are
subjected to a stronger attraction. For a like reason
the waters on the opposite side are less strongly
attracted than the mass of the earth. The result
is, that, next the moon, the waters accumulate
and form a protuberance, while, at the same
time, they accumulate and form another
protuberance on the opposite side.

If the earth and the moon remained always in
the same position, it is clear that this phenomenon
would be produced once for all. The ocean
would experience a sort of swelling on the side
next the moon and on that opposite; whilst, on
the lateral portions, the level of the waters
would be lowered.
But the earth is revolving upon her axis all
the while she is being thus acted on by the
moon, and therefore the liquid swelling takes
place successively at different points of the
earth's surface. Every instant the ocean tends
to swell, both on the side next the moon and on
the opposite side. But it is clear that the earth,
by turning, tends to drag away with it the liquid
protuberance which is formed in the direction
of the moon. It is dragged away, in fact; it
disappears little by little in proportion as it is
carried away by the moon; it is re-made again at
the same time at other points, to be carried away
and disappear in turn. And so on, continually.

It follows that the liquid swelling is never
exactly in the direction of the moon, or on the
moon's meridian. As it is always being carried
forward by the earth's rotation, it exists in
reality a little further off, beyond the direction of
the moon, past her meridian. The very friction
of the waters in the basins of the seas, by checking
the progress of the tidal wave, tends to
maintain this obliquity of the position of the
liquid protuberances in respect to the moon.

Now, when you want to slacken the revolution
of a wheel, what course do you adopt to effect
that object? You make use of friction. You put
on a drag, or apply a brake, to prevent your
carriage from running too fast down hill. Exactly
so, the moon has clapped an ever-acting brake on
the earth's rotation. The tidal wave runs
contrary to the direction of that rotation. It chafes,
and rasps, and wears away not only the shores,
but also the bottoms of our shallow seas. It
applies continual friction, tending to impede the
spinning of our planet as she flies round her
orbit. It affords the moon a handle by which to
pull the earth continually back, and inevitably
diminish the speed of her rotation.

There is another curious consideration
connected with the subject. Whatever destroys, or
tends to destroy, motion, thereby generates heat.
The tidal wave, therefore, generates heat, which
is partly radiated into space, and so lost to us.
This incessant loss of heat is as continually
supplied by the earth's rotation. The heat so
generated is one of the few exceptions to the
derivation of all heat, directly or indirectly, from the
sun. Supposing, as Professor Tyndall puts the
case, that we turn a mill by the action of the
tide and produce heat by the friction of the
millstones; that heat has an origin totally different
from the heat produced by another pair of
mill-stones turned by a mountain stream. The former
is produced at the expense of the earth's rotation;
the latter at the expense of the sun's heat,
which lifted the mill stream to its source.

No doubt such an influence bears on the
permanence of our present terrestrial conditions.
A change in them is going on. The check,
however, thus put on the earth's revolution need
cause no serious alarm, either to the existing
generation, or to those who are soon to step into
our shoes. True, in consequence of this
preventive check, the length of our day is continually
augmenting; because our days are the
consequence of the earth's rotation on her axis.
But the argumentation itself amounts to no
more than one second of time in the course of
one hundred thousand years.

There are eighty-six thousand four hundred
seconds in a day of the current Anno Domini.
If it requires one hundred thousand years for the
day (in consequence of the earth's more sluggish
rotation) to increase by the eighty-six thousand
and four hundredth part of its length, it will
take eight thousand six hundred and forty
millions of years to cause that rotation to cease
altogether, supposing the slackening of its speed
to continue under the same conditions. Will it
ever cease? Will the earth ever come to a standstill,
as far as her rotation is concerned? No;
she will not.

Her rate of spinning is gradually slackened,
because she spins faster than the moon, who
thereby raises the waters into a heap, converting
them into a brake or drag. But when once the
earth spins no faster than the moon, she will
always have the same hemisphere turned towards
her satellite; the liquid protuberance will be no
longer carried forwards, and the moon will have
no further hold whereby to check the earth's
rotation. The period of the earth's rotation
would then coincide with that of the moon's
revolution round the earth. In short, the earth,
at last, would constantly turn the same face to the
moon, exactly as the moon turns the same to us.

It is only natural to suppose that the very
same cause has produced the singularity which
we observe in the movements of the moon. If
she always turns the same face to the earth, the
cause ought to be analogous to that now
submitted to your consideration.

But things may not even go so far as that. As
time slips away (and it requires a great many
ages to realise the circumstances alluded to),
the earth's temperature is expected gradually to
drop. The waters of the ocean may be converted
into ice; with no more water there will be no
more tides; the cause of the slackening of the
rotatory movement will disappear, and the earth
will thenceforth continue to turn with a constant
velocity.

The exact amount of the slackening is not yet
known; it is ascertained approximately only by