GRAVITY MEASUREMENTS. 
55 
to continental stations where the adjacent regions varied con¬ 
siderably in density. The approximate average elevations 
within a radius of 100 miles were estimated in this manner 
for the stations in these series, taking mean values for the 
density of land (2.56) and of sea water (1.03). The observed 
values of gravity were then reduced to sea level by both Bou¬ 
guer’s and Faye’s methods,* and the results compared with 
those computed with the same theoretical formula as before. 
The residuals were averaged for each geographical group of 
stations, including those of the past season, and they are given 
for each method of reduction plotted to scale in Fig. 2. This 
diagram shows graphically the excess of gravity on islands 
and the defect on continents when the sealevel reduction is 
Asia. Pacific Ocean. North America. Atlantic Ocean. 
Australia. 
V' 
Africa. 
Fig. 2.—Differences observed minus computed gravity (in dynes) from C. and G. S. 
observations. The figures in parenthesis show the number of stations included. 
made by Bouguer’s formula, as has been pointed out from 
other series of observations. These apparent anomalies 
very largely disappear, however, on applying the theory of 
Faye. Important light would be thrown on this subject by 
the measurement of gravity on the open sea, w 7 hich may 
some day be accomplished by working on large fields of ice. 
The true reduction of gravity observations to sea level 
would therefore appear to be one that would take account 
* In both of these methods as here used corrections are applied for eleva¬ 
tion, topography, and surface attraction, and they differ only in the last. 
In Bouguer’s formula the vertical attraction of the mass above sea level is 
subtracted, while in the method here called Faye’s reduction allowance 
is made for the attraction of a plain whose thickness represents the lack 
of local compensation, as already explained. This latter reduction has 
not heretofore been developed in any definite mathematical form. 
8—Bull. Phil. Soc., Wash., Vol. 13. 
