The Democratic advocate. [volume] (Westminster, Md.) 1865-1972, 1882-02-25
Our 0ln.
The Story of the Tides.

The Story of the Tides.
What they Tell of the Growing Length of tn
Day and of the Birth of the Moon.
From scientific point of view the worl
done by the tide is of unspeakable impor
tanee. Whence is this energy derived witt
which the tides do their work ? If tn,
tides are caused by the moon, the energ,
they possess must also be derived from tn,
moon. This looks plain enough, but un
fortunately it is not true. Would it b,
true to assert that the finger of the rife
man which pulls the trigger supplies th'
energy with which the rife bullet is ami
mated? 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 man.
ner 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 a
comparison between the earth rotating on
its axis and the fy-wheel of an engine
The fy-wheel is sort of reservoir into
which the engine pours its power at each
stroke of the piston. The various ma
chimes in the mill merely draw of the pow
er from the store accumulated in the fly
wheel. The earth is ike a gigantic fly
wheel detached from the engine, though
still connected with the machines in the
mill. In that mighty fy-wheel stupen-
dous quantity of energy is stored up, and
a stupendous quantity of energy would be
given out before that hy-wheel would come
TO rest The earth's rotation is the reser-
voir 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 fiy-wheel of the engine in very
important point. As the energy is with
drawn from the fy-wheel by the machines
in the mill, so it is restored thereto by the
power Of the steam-engine, and the fy
runs uniformly But the earth is merely
the fy-wheel without the engine. When
the work done by the tides withdraws ell
ergy from the earth, that energy is never
restored Ii, therefore, follows that the
earths rotation must be decreasing. This
leads to a consequence of the most wonder
ful importance. Il 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 breyity. 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 day a
hundred years ago. Even in thousand
years the change in the length of the day
Is only fraction Of second But the
importance arises from the fact that the
change, slow though it Is, lies always in
one direction. The day IS continually in
ereasing. In millions Of years the aecum-
ulated effect becomes not only appreciable
but even Of startling magnitude.
THE CHANGE IN THE DAYS LENGTH.
The change in the length of the day
must involve a corresponding change in the
motion Of the moon. If 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 tormented
by the moon, so il strives to drive away its
prosecutor At present the moon revolyes
round the earth at distance of about 240, :
600 miles. The reaction Of the earth tends
to Increase that distance and to force the
moon 10 revolve in an orbit which Is con-
tinnally 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 proba-
bly, 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 re-
mote epoch, which was crisis in the his
tory of our system. Ii must have been at
least 81, 0,0O0,0O0 years ago. Ii may have
been very much earlier. This crisis was
the interesting occasion when the moon was
born. The length Of the day was only a
very few hours, If We call it 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 be-
come still less and finally disappear alto
getherl 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 that there is protu-
beranee at the equator, so that, as our
school books tells Us, the earth is shaped
like an orange Ii is well known that this
protuberanee is due TO the rotation 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
protuberanee. If, however, the rate of ro-
tation exceeds a certain limit the equatorial
portions of the earth could no longer cling
together The attraction which unites
them would be overcome by centrifugal
force, and a general break up would occur.
1t can be shown that the rotation of the
earth when on the point of rupture CORES
ponds to length of the day somewhere
about the critical value of three hours,
which we have already adopted. 1t is,
therefore, impossible for us to suppose a
day much shorter than three hours.
THE MOONS HISTORY.
Let us leave the earth for few minutes
and examine the past history of the moon.
We have seen the moon reyolyes around the
earth in an overwhelming orbit, and conse-
quently the moon must in ancient times
have been nearer the earth than it is now.
No doubt the change Is slow. There is
not much difference between the orbit of
the moon a thousand years ago and the
orbit in which the moon is now moving.
But when we rise to millions of years the
difference becomes very appreciable. Thirty
or forty millions of years ago the moon was
much closer to the earth than it is at pres
ent very possibly the moon was then only
half its present distance We must, how
ever, look still earlier, to certain epoch
not less than fifty millions of years ago.
At that epoch the moon must have been so
close to the earth that the two bodies were
almost touching. Everybody knows that
the moon now revolyes around the earth in
period of twenty-seven days. The period
depends upon the distance between the
earth and the moon In earlier times the
month must have been shorter than our
present month. Some millions of years
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