GRAVITY MEASUREMENTS. 
57 
2.67,2.57, 2.58, 2.61,2.61, 2.63, 2.71. The mean value, 2.615, 
has been adopted. 
The difference in gravity at the base and summit of a 
mountain is, by Bouguer’s formula, 
3 d 
H 3 g 8 
considering the upper station as on an infinite horizontal 
plain, the lower station as unaffected by the attraction of the 
mountain, and neglecting the insignificant effect of change 
in centrifugal force. Here H is the difference in elevation 
of the two stations (2,452 meters), r the radius of the earth, 
d the density of the mountain (2.615), and 4 the mean density 
of the earth. The correction to observed gravity for local 
topographical irregularities (or departure from the condition 
of a horizontal plain) was computed for these two stations 
as described in discussing the reduction to the sea level and 
found to be for Pikes Peak + 439, and for Colorado 
Springs + 27 (using the meter as unit of length). The 
observed values of gravity were for Pikes Peak 978.940, and 
for Colorado Springs 979.476, to which must be applied the 
above corrections for topographical irregularities to obtain 
dg for the above formula, which then becomes : 
dg = (979.476 + 27) - (978.940 + 439) = 
^ 2452 — 2452; or A = 2.153 s = 5.63. 
r 2 r n 
The value for the mean density of the earth derived from 
the Pikes Peak observations is therefore 5.63. In this com¬ 
putation no account has been taken of the estimated differ¬ 
ence in density below the level of Colorado Springs. 
Quarter second Pendulum Apparatus .—With the idea of ob¬ 
taining instruments still more portable than those hereto¬ 
fore used for measuring relative gravity, there was con¬ 
structed about a year ago, as planned by Doctor Mendenhall, 
