GRAVITY MEASUREMENTS. 
47 
to omitting the correction for attraction (second term) in 
Bouguer’s formula, but not so in coast and oceanic island 
stations where the effect of the lesser density of sea water is 
taken into account. On the theory that the surface of the 
earth is in a state of hydrostatic equilibrium, M. Faye, in 
papers published in “ Comptes Rendus,” in 1880 and 1883, 
advocated that the continental attraction term in Bouguer’s 
formula be omitted, but that all local deviations of the ele¬ 
vation of the station from the general level of the land 
surface or sea bottom be allowed for, as, for instance, the 
attraction of a mountain on a station at its summit or of an 
isolated island. This subject has also been discussed by 
Stokes, Peirce, Ferrel, and others. In the recent reports on 
pendulum observations in the proceedings of the Interna¬ 
tional Geodetic Association, the reductions have been made 
by two methods—first, with Bouguer’s formula, and, second, 
omitting the attraction term and using the reduction for 
elevation, and the same plan is followed in the summary of 
results given in Table C (see p. 48). For use in Bouguer’s 
formula the mean density of the earth has been taken as 5.58. 
The surface densities used are from information kindly fur¬ 
nished by Mr. G. K. Gilbert, of the U. S. Geological Survey, 
and are based on a personal examination in many cases. 
These represent the -estimated averages for the entire mass 
above sea level. 
In order to be able to study the results more intelligently, 
the values at sea level have been compared with those com¬ 
puted by an assumed theoretical formula, 
g = 978.066 (1 + .005243 am 2 <p\ 
which is based on Clairaut’s theorem, Clarke’s figure of the 
earth, and the assumption that gravity is normal on the 
eastern coast of the United States. Finally, this table gives 
the difference, observed minus computed gravity, for both 
methods of reduction to sea level. These differences are 
shown graphically on Plate 5 for those stations near the 
39th parallel, including also two stations, San Francisco and 
Mount Hamilton, previously determined. 
7—Bull. Phil. Soc., Wash.. Vol. 13. 
