The central Presbyterian. [volume] (Richmond, Va.) 1856-1908, 1883-01-17
The Story of the Tides.
WHAT THEY TELL OF THE GROWING
LENGTH OF THE DAY AND OF THE
BIRTH OF THE MOON

BIRTH OF THE MOON.
From scientific point of view the
work done by the tides is Of unspeakable
Importance Whence is this energy de-
rived with which the tides do their
work? If the tides are caused by the
moon, the energy they posses must also
be derived from the moon. This looks
plain enough, but unfortunately it is not
true. Would it be true to assert that the
finger of the riheman which pulls the
trigger supplies the energy with which
the rifle bullet is animated? 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 a 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
engine. The fy.wheel is a sort of
reservoir, into which the engine pours its
power at each stroke of the piston. The
various machines in the mill merely
draw off the power from the store ac-
cumulated in the fy-wheel. The earth
is like gigantic fy-wheel detached from
the engine, though still connected with
the machines in the mill In that mighty
hy-wheel stupendous quantity of energy
would be given out before the hy-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 energy they draw on the
supply ready to hand in the rotation of
the earth. The earth differs from the
Hy-wheel OF the engine in a very impor-
tant point AS the energy is withdrawn
from the fy-wheel by the machines in
the mill, so it is restored thereto by the
power of the steam-engine, and the fly
runs uniformly. But the earth is merely
the hy-wheel without the engine. When
the work done by the tides withdraws
energy from the earth, that energy is
never restored. 1t therefore follows that
the earth's rotation must be decreasing
This leads to 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 effect 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 thousand years the change in
the length of the day is only q fraction of
a second. But the lmportanc, rises from
the fact that the change, slt.a though it
is, lies always in one direction The day
is continually increasing. In millions of
years the accumulated effect becomes not
only appreciate but even of startling
magnitude
The change in the length of the day
must involve corresponding change in
the motion of the moon II the moon
acts on the earth and retards the rotation
OF the earth, so, conversely, does the earth
react upon the moon. The earth is tor
mented by the moon, so it strives to drive
away its prosecutor. AT present the
moon revolves round the earth at dis
lance of about 100000 miles. The re-
action of the earth tends to increase that
distance, and to force the moon to revolve
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 projec
our view back to an incredibly remote
epoch which was crisis in the history OF
our system. 1t must have been at least
: 100000 years ago. 1t may have been
, very much earlier. The crisis was the
interesting occasion when the moon was
born. The length of the day was only a
, very few hours If we call If three hours
we shall not be far from the truth. Per.
? haps you may think that if we looked
{ back to a still earlier epoch, the day
would become still less and finally disap
2 pear altogether This is however not
f the case The day can never have been
e much less than three hours in the present
] order of things Everybody knows that
the earth is not a sphere, but that there
p is protuberance at the equator so that
t as our school books tell us, the earth is
shaped like an orange. 1t is well known
that this protuberance is due to the rota
ton of the earth on its axis. by which the
equatorial parts bulge out by centrifugal
force. The quicker the earth rotates the
greater is the protuberance. IT, how
i ever, the rate of rotation exceeds a GER