GRAVITY MEASUREMENTS. 
39 
station and the known point by hand level. In the field¬ 
work the writer had the efficient assistance of Mr. C. E. Men¬ 
denhall at fifteen stations and of Mr. S. B. Tinsley at three 
stations. At eight stations the observations were conducted 
without aid. 
To avoid objections that might be raised to swinging three 
pendulums upon a single knife-edge, and to guard against 
injury to it, an extra or standard edge was provided, upon 
which the pendulums were swung at Washington and at two 
other stations. Although the ordinary knife-edge (marked 
A I) had about four times as much use as the standard 
(marked A II), the independent determinations of the pend¬ 
ulum periods on the two knife-edges show a fairly constant 
difference, indicating no wear sufficient to materially affect 
the periods. Comparisons were made by a complete series 
of swings covering forty eight hours on each edge, and the 
mean periods of the three pendulums, reduced to standard 
conditions, are given below: 
Period 
on A I. 
Period 
on A II. 
Differences, 
A I—A II. 
Washington, D. C.. 
Washington, D. C.. 
Chicago, Illinois... 
Denver, Colorado.. 
June23-27,1894.... 
Oct. 31-Nov. 4,1894. 
Julv 11-16,1894.... 
Sept. 5-10, 1894.... 
s. 
.5007124 
7121 
6697 
8409 
s. 
.5007110 
7108 
6689 
8392 
s. 
+.0000014 
+ 13 
+ 08 
+ 17 
Mean... 
+.0000013 
The periods of the half second pendulums have been de¬ 
termined at the base station at Washington six times during 
the past year on knife-edge A I and using the system of 
eight hour swings and low pressures, with the following re¬ 
sults : * 
*The transcontinental series of observations at twenty stations was 
made between the fourth (June 23-27) and fifth (October 31-November 4) 
determinations of period at Washington given in the table. 
6—Bull. Phil. Soc., Wash., Vol. 13. 
