Evening star., 1930-06-04
SHAPE OF EARTH
CHECK IS PLANNED
Subterranean Vault Appa-
ratus Being Set Up by
Bureau of Standards.
BY THOMAS r. HENRY.

Apparatus Is being set up in the sub
terranean vault at the Bureau of Stand
ards to determine within one part in a
million the value of the constant of
gravity at Washington, to be used as a
standard for the United States, the
ultimate object being a more precise
determination of the shape of the earth.
The constant of gravity-the rate of
acceleration of falling body under the
pull of gravity-which now Is used was
established at Potsdam, Germany, more
than 25 years ago with extremely deli
cate measurements which required more
than eight years. The German 'physi-
cists determined that a falling body at
sea level at the Equator would be &C
celerated 32.0875 feet per second in o
vacuum. This value, used with math-
ematical formula, has been used as a
standard of comparison to determine
the distance of any spot from the CEN
ter of the earth.
German Calculations Questioned.
But American geophysicists are by
no means convinced that the German
of a part in a million of error. They
also believe very slight difference in
the value of the constant might make
noteworthy difference in maps of the
earth. The Bureau of Standards is un-
dertaking the work, which Is expected
to last several years, for the Coast and
Geodetic Survey. The project is under
charge of DR.. Paul r. Heyl, who in the
same underground vault has just COM
pleted a reweighing of the earth with q
greater accuracy than was ever achieved
before.
SuperficialIy, the determination of the
constant Is q simple process. Gravity
Is what makes pendulum swing. The
speed of the swings in a vacuum de-
pends on the length of the pendulum
and its distance from the center Of the
earth, where, so far as gravity Is con-
cerned, the mass of the planet acts as
If it were concentrated. The farther
from the center of the earth the shorter
the pendulum must be to swing in the
same periods.
So the constant of gravity is meas
ured by determining as precisely as
possible the length of the pendulum
and the speed of its swing at any point
on the earth's surface. If the earth
were a perfect sphere. the period of
swing of standard pendulum would
be the same e. erywhere, since every
point would be equidistant from the
| center. Actually, the distance from
either of the Poles to the center is
3,949. .790 miles, and from the Equator
1868 225 miles, according to present
measurements. Consequently, the ac-
celeration of gravity is less at the
Equator, since the attracting mass-the
mass of the earth concentrated at It's
| center-iS farther away At the poles
1t is approximately 32.2577 feet per
| second.
Earth's Shape Determined.
The value found by pendulum swings
at any point between the Equator and
the poles enables engineers to calcu-
late very accurately how far that point
is from the center, and consequently to
determine the actual shape of the earth
The value increases -millionths of a
| foot per second for each 1000 feet of
| deviation upward from sea level. The
present value used for gravity at Wash
ington, considered the base station for
the United States. is 321558, but this
is based on the German calculation of
the absolute value.