causes
the earth's rotation. Computation shows that the amount of this action
should be great enough gradually to lengthen the day, or the time
occupied by the earth in making a complete revolution on the polar
axis. The effect ought to be great enough to be measurable by
astronomers in the course of a thousand years. On the other hand, the
records of ancient eclipses appear pretty clearly to show that the
length of the day has not changed by as much as a second in the course
of three thousand years. This evidence does not require us to abandon
the supposition that the tides tend to diminish the earth's rate of
rotation. It is more likely that the effect of the reduction in the
earth's diameter due to the loss of heat which is continually going on
counterbalances the influence of the tidal friction. As the diameter
of a rotating body diminishes, the tendency is for the mass to spin
more rapidly; if it expands, to turn more slowly, provided in each
case the amount of the impulse which leads to the turning remains the
same. This can be directly observed by whirling a small weight
attached to a string in such a manner that the cord winds around the
finger with each revolution; it will be noted that as the line
shortens the revolution is more quickly accomplished. We can readily
conceive that the earth is made up of weights essentially like that
used in the experiment, each being drawn toward the centre by the
gravitative stress, which is like that applied to the weight by the
cord.

The fact that the days remain of the same length through vast periods
of time is probably due to this balance between the effects of tidal
action and those arising from the loss of heat--in other words, we
have here one of those delicate arrangements in the way of
counterpoise which serve to maintain the balanced conditions of the
earth's surface amid the great conflicts of diverse energies which are
at work in and upon the sphere.

It should be understood that the effects of the attraction which
produces tides are much more extensive than they are seen to be in the
movements of the sea. So long as the solar and planetary spheres
remain fluid, the whole of their masses partake of the movement. It is
a consequence of this action, as the computations of Prof. George
Darwin has shown, that the moon, once nearer the earth than it is at
present, has by a curious action of the tidal force been pushed away
from the centre of our sphere, or rather