GEOLOGY: W. BOWIE 
171 
begun per second is, in the case supposed, 5 = P + F = w-7-r + |5, and so ^ B = n 
-i- T. Hence (350 ^ d) n g = n ^ t, oi g = (6 ^ 350r). But when F = ^ B, {d ^ t) 
= loge 2, and so g = 0.00 2 nearly. 
6 Hall, E. H., Boston, Proc. Amer. Acad. Arts and Sci., 50, 1914, (67-103). 
THE GRAVIMETRIC SURVEY OF THE UNITED STATES 
By William Bowie 
DIVISION OF GEODESY, U. S. COAST AND GEODETIC SURVEY 
Communicated by W. M. Davis, January 12, 1917 
The gravimetric survey of the United States really began in 1890 
with the introduction of the Mendenhall one-half second invariable 
pendulum. Previous to that date, 13 stations had been established 
but in that older work pendulums were used which gave inaccurate 
results, as later work showed. We shall not, therefore, consider the 
gravity results obtained before the use of the Mendenhall pendulum. 
This pendulum consists of a bob and stem with a suitable head into 
which is set an agate plane which rests on a knife edge of the same 
material fastened to the pendulum case. The various parts of the ap- 
paratus are illustrated and described in reports of the Coast and Geo- 
detic Survey.^ The Mendenhall pendulums are used to determine the 
difference in the intensity of gravity at two stations. 
The probable error of the value of gravity at Washington, deter- 
mined from Potsdam by the relative method, is =t= 0.001 dyne. The 
probable error of a station in the United States, other than that at 
Washington, is about =b 0.002 dyne. This is about one part in one- 
half million. The error of the absolute value at Potsdam enters all 
other values of gravity based upon the Potsdam system. 
Between the years 1891 and 1907, 47 stations were established in the 
United States, while since January, 1909, 212 additional stations have 
been established, making 259 in all. The Coast and Goedetic Survey 
has planned to continue its gravimetric survey for an indefinite period, 
with a view to covering the large areas now lacking in stations, and also 
local areas where there are special problems to be investigated. 
There are two immediate purposes to be served in carrying on, this 
work. First, to collect data from which more accurate values may be 
obtained for the flattening of the earth and for the terms in the gravity 
formula. Second, to obtain values of the intensity of gravity at labora- 
tories as these values are needed in certain physical and chemical work. 
Another important use to which the gravimetric survey may be put 
is in researches into the subject of isostasy. While an old subject^ 
isostasy has only recently become a vital matter to be considered in 
