Vol. 6, 1920 
GEOPHYSICS: W. BOWIE 
549 
mountain masses and other topographic features. Some of the determina- 
tions of the figure of the earth were based on all of the observed geodetic 
data, that is, triangulation and connected astronomic stations, and other 
determinations were made to depend only on those data from which sta- 
tions with large deflections of the vertical had been eliminated. 
In the late fifties of the past century, Archdeacon John H. Pratt an- 
nounced the theory that the geodetic data seemed to indicate that under 
mountain masses there were deficiencies of matter approximately equal 
in quantity to the masses that were above sea level. This theory was 
frequently commented on and studies were made to intepret certain geo- 
logical features by means of it. 
In 1889, Major C. B. Button, in an address before the Philosophical 
Society of Washington, entitled "On some of the greater problems of 
physical geology,"^ applied the term "isostasy" to this theory of equal 
pressure, that the mountains are accompanied by deficiencies of mass 
under them and that there is an excess of matter under the oceans. 
About fifteeh years after Dutton gave prominence to this theory, Prof. 
John F. Hayford, at that time Chief of the Division of Geodesy of the 
United States Coast and Geodetic Survey, began investigations which 
showed rather conclusively that the material above sea level is counter- 
balanced to a marked degree by a deficiency in density of material in the 
column directly under the exposed material. This column, which is 
affected by this decreased density, was found by Hayford to extend to a 
depth of approximately 122 kilometers, if the compensation is assumed to 
be uniformly distributed. Hayford also found that the column extending 
approximately 122 kilometers below the bottom of the ocean has a density 
in excess of normal that will balance approximately the deficiency of 
matter in the mass of water directly above the column. 
This condition of approximate equilibrium in the earth's material 
at a depth of approximately 122 kilometers was applied by Hayford in 
determining the figure of the earth from geodetic data in the United 
States. The results appear in his report published in 1909,^ which gives 
dimensions for the earth that are believed to be of a higher order of ac- 
curacy than any previously arrived at. This is because the theory of 
isostasy was considered in this work. It is hoped that in the future the 
theory of isostasy can be considered in deriving a figure of the earth from 
all available triangulation data in the world. 
In order that this figure of the earth may be computed it will be neces- 
sary to have some central office to guide the investigators of the several 
countries who may assist in the work. Undoubtedly this central office 
should be under the direction of the Division of Geodesy of the Inter- 
national Geodetic and Geophysical Union, although the central office 
itself might be located in a geodetic organization of any one of the coun- 
tries adhering to the union. 
