The Wellington enterprise. [volume] (Wellington, Ohio) 1867-188?, 1882-07-05
THE STORY OF THE TIDES.

THE STUht Of THE TIDES.
What They Tell of the Growing Length
of the Day and of the Birth of the
Moon.
From scientific point of view, the
work done by the tides is of unspeaka-
BILE Importance. Whence is this ener-
gy derived with which the tides do their
workp If the tides are caused by the
moon, the energy they possess must
also be derived from the moon This
looks plain enough, but unfortunately
it is not true. Would it be true to as
sert that the finger of the rifeman
which pulls the trigger supplies the en-
ergy with which the rife-bullet is ami
matedp OF course it would not. The
energy is derived from the explosion
of the gunpowder, and the pulling of
the trigger is merely the means by
which that energy is liberated. In
somewhat similar manner the tidal wave
produced by the moon is the means
whereby part of the energy stored in
the earth is compelled to expend itself
in work. Let me illustrate this by
comparison between the earth rotating
on its axis and the fy-wheel of an en-
gine. The fy-wheel is a sort of reser-
voir into which the engine pours its
|
The various machines in the mill mere
ly draw off the power from the store
accumulated in the fy-wheel. The
earth is like a gigantic fy-wheel de-
tached from the engine, though still con-
nected with the machines in the mill.
In that mighty fy-whee1 a stupendous
quantity Of energy is stored up, and
stupendous quantity of energy
would be given out before that fly
wheel would come to rest. The
earth's rotation is the reservoir from
whence the tides draw the energy they
require for doing work. Hence it is
that though the tides are caused by the
moon, yet whenever they require ener-
gy they draw on the supply ready to
hand in the rotation of the earth. The
earth differs from the fy-wheel of the
engine in very important point. As
the energy is Withdrawn from the fly
wheel by the machines in the mill, so it
is restored thereto by the power of the
steam-engine, and the fly runs uniform
ly. But the earth is merely the fly
wheel without the engine. When the
work done by the tides withdraws ener-
gy from the earth, that energy is never
restored. 1t, therefore, follows that the
earth's rotation must be decreasing.
This leads to q consequence of the most
wonderful importance. 1t tells us that
the speed with which the earth rotates
| on its axis is diminishing. We can
state the result in a manner which has
the merits of simplicity and brevity.
The tides are increasing the length of
the day. At present, no doubt, the eff
| feet of the tides in changing the length
of the day is very small. A day now is
not appreciably longer than a day a
hundred years ago. Even in a thou
sand years the change in the length of
the day is only a fraction of q second.
But the importance arises from the fact
that the change, slow though it is, lies
| always in one direction. The day is
continually increasing. In millions of
years the accumulated effect becomes
not only appreciable, but even of start
ling magnitude.
The change in the length of the day
must involve corresponding change in
the motion of the moon. If the moon
acts on the earth, SO, conversely, does
the earth react upon the moon. The
earth is tormented by the moon, so it
strives to drive away IFS prosecutor. At
| present the moon revolveS round the
| earth at a distance of about 240, 900
miles. The reaction of the earth tends
to increase that distance, and to force
the moon to revolye in an orbit which
is continually getting larger and larger.
As thousands of years roll on, the
length of the day increases second by
second, and the distance of the moon
increases mile by mile. A million years
ago the day, probably, contained some
minutes less than our present day of
twenty-four hours. Our retrospect does
not halt here; we at once project our
view back to an incredibly remote
epoch which was crisis in the history
of our system. 1t must have been at
least 5O,000,O00 years ago. 1t may
have been very much earlier. This
crisis was the interesting occasion
when the moon was born. The
hours. If we call it three hours
we shall not be far from the
truth. Perhaps you may think that il
we looked back.to still earlier epoch,
the day would become still less, and
finally disappear altogether! This is,
however, not the case. The day can
never have been much less than three
hours in the present order of things.
Everybody knows that the earth is not
sphere, but there is protuberance al
the equator, so that, as our school-books
tell us, the. earth is shaped like ar
orange. 1t is well known that this pro
tuberance is due to the rotation of the
earth on its axis, by which the equa-
torial parts bulge out by centrifugal
force. The quicker the earth rotates
the greater is the protuberance. If
however, the rate of rotation exceeds {
certain limit, the equatorial portions O
the earth could no longer cling togeth
er. The attraction which unites then
would be overcome by centrifugal force
and a general breakup would occur
1t can be shown that the rotation of tn,
earth when on the point of rupture cor
responds to length of the day some
where about the critical value of thre,
hours which we have already adopted
| 1t is therefore impossible for Us to sup
pose day much shorter than thre,
hourA 91