Voiv. 6, 1920 
GEOPHYSICS: W. BOWIE 
55-1 
pendulums approximately one meter in length, to get the relative value 
of gravity or the difference in gravity between two places, one of which 
was the base station. These results were not particularly satis actory be- 
cause the pendulums were swung in the air with rather poor temperature 
control and the effect of the flexure of the support due to the swinging pen- 
dulum was not considered, or, if it was, was not accurately determined. 
About 1885, Von Sterneck devised an apparatus which consisted of 
a quarter-meter pendulum of the invariable type, swung in a case from 
which the air could be almost entirely exhausted. With this apparatus, 
the difference in gravity between two stations could be determined with 
a high degree of precision. There have been modifications of the Von 
Sterneck pendulums, but only in minor features, the general principle of 
the apparatus remaining unchanged until this time. 
With the short pendulums of the Von Sterneck type it was possible to 
establish a station in a very short time and at a very low cost. The re- 
sult has been a great stimulation of effort in this branch of geodetic work. 
Most of the countries of the world carrying on geodetic operations have 
adopted this type of pendulum and many stations have been established. 
In 1908 there were 47 modern gravity stations in the United States, 
while to-day there are 276 such stations. 
Gravity determinations have a number of uses ; one is to furnish a formula 
by which the value of gravity can be computed for any latitude for use in 
the physical and chemical sciences; another, to furnish data by which 
the shape of the earth may be determined; and another, to furnish data 
for making studies of the theory of isostasy or the deviation from normal 
densities of matter in the outer portion of the earth. 
As a result of the application of the theory of isostasy to the gravity 
determinations in the United States, che shape of the earth, or rather the 
amount of its flattening, has been determined by investigations made 
by the Coast and Geodetic Survey and published in 1917 as Special Pub- 
lication No. 40 of that Bureau. The reciprocal of the flattening was 
found to be 297.4. This value is probably the most reliable one in ex- 
istence to-day. There were also derived from this investigation new con- 
stants for the gravity formula for computing the value of gravity at any 
latitude. This formula was adopted by the U. S. Weather Bureau and 
by the Smithsonian Institution in the preparation of the latest edition 
of the Meteorological Tables. In that investigation was also determined 
the depth of compensation; that is the depth at and below which the 
materials of the earth are supposed to be in a state of equilibrium. This 
depth was found to be 96 kilometers, which agrees very closely with the 
value derived by Hayf ord in his determination of the figure of the earth at 
stations in mountainous regions only. It is necessarily true that a depth 
of compensation can be derived from data in elevated regions with greater 
certainty than where the land is low. The higher land is liable to be freer 
