Atlanta tri-weekly journal. [volume] (Atlanta, Ga.) 1920-19??, 1922-10-03
LAND HAS TIDES LIKE SEA
(By Science Service)

The sea has no monopoly on the tide
business. Walter D. Lambert, of the United
State coast and geodetic survey, points out
that the land rises and falls just like the
waters of the big pond.
''The same attraction Of the sun and
moon which raises tides in the ocean," he
says, "must also raise tides in the so-called
solid earth itself, for an absolutely rigid
and unyieldinng body is merely a mathe-
NAUTICAL fiction. In observing tides we nec-.
essarily refer them to the earth, that
what we get by observation is the differs
ence between the tide in the water and the
| earth tide at the same point. If we could
determine from a rigorous mathematical
theory what the water tide would be on the
hypothesis of an absolutely unyieldng earth.
we could then compare the tide actually,
observed with the theoretical tide and the,
difference would represent the yielding Of,
the earth earth-tide. Only in & few
| simple cases can we predict the water tide
by mathematical theory alone One such
| case is that Of the called long period
| tides, tides whose periods are about fort
night or month. The observed long-period
| tides are found to have about two-thrds of
their theoretical values, an indication that
| the earth yields somewhat more than a
| globe of steel would do.
| Another case where the tide may be
predicted from theory alone is that of the
/
| not too shallow body of water. Such a body
| might be anything from trough a hundred
| feet long or so up to a body OF water like
Lake Superior. The tidal forces cause small
| changes in the direction of the plumb line,
| and the water surface adjusts itself so as
| to remain always perpendicular to the in-
/ Simultaneous direction of the plumb line di-
/ rectly. It's motion is very small and an APP
| psratus called the horizontal pendulum has
/ been used to render the motion accessible
to observation. When water surface is
/ observed, it is more satisfactory to use an.
| artificial body of water than natural One;
| this was done by Drs. Nicholson and Gale.
4 wh1 observed the minute tides In a partly
i fillsi horizontal pipe some 500 feet long,
/ These tides were measured by very deli.
cate apparatus perfected by Professor Michel
son that depends on the interference Of
| light waves from two different sources and
| Is called the inteferometer. The work
! OF these two scientists represents the most
i accurate measurement Of earth tides that
! has yet been made"
"There are difficulties in interpreting.
| some Of the results of observation on the
diurnal or semi-diurnal earth tides as OBJ
| served with a horizontal pendulum or in q Il
horizontal pipe" says Mr.. Lambert. OBJ
servations made at several places not far
from the Baltic Sea seemed to indicate
! that the earth is more rigid in an east+
| and-west than in a north-aCd-south direc-
! ton. Many explanations have been given of
| this phenomenon, all but one Of them more
interesting than satisfying. Inc only SooQ ,
explanation is that the effec: is due to the
ocean tides even though the ocean may be
many miles away. AS the water in the
ocean moves in response to the tidal forces
it's attraction on the plumb line varies;
furthermore the shifting load of water
causes variable tilt in the earth's crust
and this too affects the plumb line by af-
fecting the point of support.
The ocean tides are known only close to
land; the tides in the open sea are mat
| tiers of inference, put calculations based on
the best data obtai1able indicate that the
supposed difference between the eas-west
rigidity and the north-south rightly noticed
in Europe may be adequately explained as {
the effect Of oceanic tides Recent work
on earth tides has tended towards hisher
values for the rightly of the earth One
recent determination would make the eff
| fective rigidity of the eartn as whole ABOUT !
| twice that of steel. The range of the semi (
! diurnal lunar earth tide would be about ten
| inches at the equator; the minimum range
of the diurnal earth tide, which occurs IN
! latitude As degrees, would be about the
| same and the range Of the solar tide al tne.s
equator would be about half as much. >