Omaha daily bee., 1916-08-04
Shrinkoge Speeds
Earth's Rotation
By GARRETT P. SERVISS.


earth is increasing in weight, or that
its period of rotation is either increas-
ing or decreasing. But there are good
reasons for inferring that its weight
continually increases, and that, If
there is change in its speed of ro-
tation, the change is in the direction
OF loss rather than of gain.
Some observations have led to the
rather startling conclusion that the
rotation is irregularly variable. Prof.
I. A. Young put the matter in this
form: There are suspicious indica-
tons that Greenwich noon has, at IT.
regular intervals of from thirty to
fifty years, sometimes come too early
by as much as four or five seconds,
and at other times fallen as much be-
hind"
If the supposed irregularity
amounted to hours instead of seconds
it would be very serious thing for
US, but, being so slight, if it exists
at all, the variation could only affect
our standards of time In that way
it would be important, but not other
wise, unless the irregularity should
increase.
But the majority of observations
show no measurable variation. There
are three principal causes to which
change in the earth's speed of rota-
rtion might be due, but they do not all
act the same way.
The first Of these causes, and one
that tends to hasten the rotational
velocity, is the slow shrinkage of the
earth's bulk, due to the gradual cool
ing off of its interior. The core of
the earth must be very hot, on &C
count of the enormous pressures ex-
isting there, but the calculations con-
corning the rate at which this internal
heat is escaping are discordant, be-
cause of the difficulty of obtaining
any experimental knowledge of the
subject.
We certainly are not aware of any
heat gushing up out Of the earth ex-
cept in the neighborhood of volcanoes
and hot springs. But if heat does
regularly escape from the interior OF
the earth, then, inevitably, the globe,
as whole, must shrink.
Indications of this shrinkage are
given by great earthquakes, caused
by the settling down of the rock
strata to keep in firm touch with their
underpinning. Such shrinkage, as /
have already said, would tend to in
crease the speed OF rotation.
On the other hand, there are two
causes which must tend to retard the
rotation. One of these is the friction
of the tides, and the other is the in
crease of the earth's bulk and weight
through the influx of meteoric bodies
from outer space. Tidal friction is
due to the fact that the earth rotates
on its axis faster than the moon
which is the principal agent in raising
the tides-travels round the earth.
The attraction Of the moon draws
the ocean waters into tidal
waves or protuberances, which
would remain directly under the
moon if the earth did not rotate
any faster than the moon moves;
but since the rotation is relatively
rapid, the solid ball of the earth
tends to revolve inside a kid of
blanket brake, composed of the
oceanic water held, so to speak, by
the moon.
Thus the tides appear to have
general advance westward, while the
earth turns eastward, and the effect
resembles "a sliding friction
against the water. So much import
ance has been attached to tidal fric-
ton by some astronomers that it has
been regarded as very effective
agent in the evolution of worlds and
of solar systems.
But these great effects are Sup
posed to have been produced ages
ago, when the entire globe was in
plastic state, while the tides of to-
day, being notable only in the shall
low films of oceanic water, produce
relatively insignificant results.
The other cause (the influx of
meteors) which combines with
tidal friction to retard the earth's
rotation is probably even less effec-
tive at the present time, although
its results could possibly be meas
ured if we had sufficiently delicate
tests to apply. We know that solid
bodies of unknown origin are con-
tinually falling upon the earth.
Some of them, the so-called
meteorites, are quite massive, occa-
sionally weighing several tons.
There are magnificent specimens
in the American Musemum of Nat
ural History and in other museums.
But in addition to these huge
masses, and in the aggregate more
important than they, are the small
meteors or shooting stars" which
are constantly entering the earth's
atmosphere and being burned up
there through the heat of friction.
1t has been estimated that not
less than 100000 of these small
meteors enter the atmosphere
every day. But since, individually,
their weight may not exceed, on
the average, more than a few
grains, the total amount of matter
that they add to the earth must be
comparatively very small. But
there may have been times when
the earth was the meeting point of
great flocks of meteors and when
the downfall was considerable.
Indeed, according to the plan
etesimal hypothesis" the earth was
formed in some such way as that.
But have little faith in this
hypothesis as applied to the origin
of the earth.
1t is generally believed that the
small meteors which appear peri
odically in showers" are the scat
tered remains of comets. All Of
these cometary meteors are so min
ute that they are entirely con-
sumed before reaching the ground