56 
PUTNAM. 
not only of the attraction of the matter above the sea level, 
but also of the general differences of density below, as well 
as of the local lack of compensation, if the latter can be 
proved to have a systematic relation to the topographical 
situation of the point of observation, as is strongly indicated 
by the results of the past season. With such a reduction it 
is quite possible that the so-called anomalies of gravity will 
largely disappear, and that the distribution of gravity on the 
earth’s surface will be found to follow Clairaut’s law suffi¬ 
ciently closely to greatly enhance the value of pendulum 
observations in relation to the problem of the earth’s figure. 
The most distinguished of living geodesists recently wrote: 
“ Pendulum measurements will become a most important 
means of help, not only to geodesy, but also to geology. The 
widest possible extension of pendulum observations is in the 
highest measure to be desired for both sciences.”* 
Mean Density of the Earth from Pikes Peak Pendulum Ob¬ 
servations .—A value for the mean density of the earth has 
been deduced from the gravity measurements made on the 
summit of the peak and at Colorado Springs, near the base. 
This method of comparing the mass of a mountain which 
may be estimated with the unknown mass of the earth has 
been used on several previous occasions, but not, it is be¬ 
lieved, under as favorable conditions as the present, for in 
this case contour maps of the peak and surrounding county 
were available, and the mountain itself was found to be com¬ 
posed of rock of an unusually uniform density. Mr. Whit¬ 
man Cross, of the U. S. Geological Survey, who has studied 
the geology of the vicinity, furnished data in regard to the 
specific gravity of the rock of which the peak is composed. 
Twelve specimens determined under the direction of Mr. 
Gilbert gave results as follows: 2.57, 2.60, 2.61, 2.61, 2.62, 
* “ Nach dem Yorstehenden wird die Pendelmessung nicht nur fur die 
Geodasie, sondern auch fiir die Geologie als ein ausserst wichtiges Hiilf- 
smittel auzusehen sein. Moglichste Ausbreitung der Pendelmessungen 
ist fiir beide Wissenschaften in hohem Maasse erwiinscht. ” “ Die Schwer- 
kraft im Hochgebirge.” Prof. F. R. Helmert, Berlin, 1890. 
