Evening star., 1938-11-29
Of Stors, Men
And Atoms
Notebook Of Science Progress
In Field, Laboratory
And Study
By Thomas r. Henry.

BY Thomas r. Henry.
The United States Coast and Geodetic
Survey now is in position to re-detsr-
mine "the shape OF the EARTH"
In its files are an enormous accumula-
ton Of accurate distance measurements.
They cover more than 1900 square miles
of the North American continent. BY
means Of these 1t will be possible to cal
culate the figure of the planet with an
hitherto unapproached mathematical &C
curacy.
This is the ideal "oblate ellipsoid"
figure which the planet would present If
Its surface was perfectly smooth. The
new possibility is made known by DR..
William Bowie of the Coast and Geo.
detic Survey staff in the December issue
of the Scientific Monthly issued by the
American Association for the Advance
ment of Science.
Over the United States has been
thrown vast "triangulation net" with
distances between points determined
with extreme precision by the ordinary
methods Of the surveyor. These have
been checked by astronomical observa-
tions of latitude and longitude. From
the minute differences between the two
it will be possible, after vast amount
of mathematical calculation, to con-
suuct with extreme precision the ideal
ellipsoid.
The Coast and Geodetic Survey, DR..
Bowie says, no doubt will undertake this
task as soon as funds and personnel are
available
In order to derive perfect figure of
the planet, he says, 1t would be neces-
sary to have similar measurements COW
ering the entire surface. With abundant
data for any one large area, however, it
will be possible to reach an approxima-
ton which will differ from the true figure
by very minute amount. It will GER
tainly be much more accurate than the
international standard used at present.
Meanwhile the American measure
ments are being continued so that the
mathematicians can be given the utmost
possible data when they actually start
work.
The present standard figure of the
earth" was adopted by the International
Geodetic Association in 1924. This
mathematical oblate ellipsoid has q
radius OF 1868 "a statute miles from its
center to point on the Equator and of
394999 to either of the poles. The one
distance is 1885 miles shorter than the
other, representing the degree of fiat
tening, or 'oblateness" at each pole.
I'm deriving new figure Of the
earth," says DR.. Bowie, "the mathema-
tician uses the difference In the latitude
and long1tudes Of the triangulation sta
tons as derived from astronomical OBJ
servations and as derived from the COM
putations of the triangulation. If there
were no irregularity In the actual sur
face Of the earth and there were no hori-
zontal irregularities in the distribution
of the mass Of the earth, the differences
Of the astronomic and geodetic positions
of the stations would be due solely to ac-
cidental errors Of the triangulation and
astronomical measurements and to the
deviation from the truth of the adopted
figure Of the earth used III making the
geodetic computations. Under these
conditions the dimensions of the clip
sold derived for the area of some one
country, such as the United States,
would be about the same as those de-
rived from the data in other large areas.
But the irregular surface Of the
earth has the effect of influencing the
plumb line direction to which all astro-
nomical observations are referred. A
plumb line will be DETECTED to the west
at stations to the east and close to
mountain range. At stations on the
coast of continent the DETECTION will
ordinarily be toward the land and away
from the ocean. Even low ranges Of
hills defect the plumb line from the di-
rection that is normal to the ellipsoid.
There is still another disturbing in
fluence-the compensation within the
outer shell of the earth of the topog-
raphy. Under the continents the density
Of the shell materials is less than nor
NAL while under the oceans it is greater
than normal. The normal densities are
for the shell under the coastal plane.
This shell within which the compensat-
ing excesses and deficiencies exist extends
to depth of about go miles below sea
level. The higher the elevation the
greater is the deviation from normal
density in the shell below. It has been
found that the pressure exerted by any
large portion of the shell on the 60-mile
surface is practically the same as that
for any other large portion of the shell
of the same horizontal extent. This con-
dition is termed isostasy.' This con-
dition must be taken into account If the
best results are to be obtained in the der
ivation of the dimensions of the earth
from astronomical and geodetic data"
With this iso0tatic equilibrium taken
into account, DR.. Bowie says, the dimen-
sions of the earth computed from meas
urements on one continent should be
exactly the same as those computed from
measurements on another continent.
Actually there probably will be slight
error since the distribution of compen-
sating densities In the shell of the earth
is somewhat irregular. The error, how
ever, will be very small.
Cites Dilemma in