32 
PUTNAM. 
forges, in France, a little later. At present almost every 
nation carrying on geodetic surveys has taken up this sub¬ 
ject to a greater or less extent. A few years since, Dr. T. C. 
Mendenhall designed for use on the Coast and Geodetic 
Survey a half second pendulum apparatus differing in im¬ 
portant respects from that used in Austria, although previous 
to that time valuable experimental and field research had 
been carried on for the Survey in this line by Mr. Charles S. 
Peirce, Mr. E. D. Preston, and others, principally in connec¬ 
tion with various foreign expeditions. During the past year 
an extensive series of gravity measurements was made in the 
United States with the half second pendulums, employing 
convenient methods not heretofore thought applicable to 
such small apparatus, and in addition instruments more 
portable than any before used, were tested. It is the pur¬ 
pose of the present paper to briefly describe this work and 
to give a summary of the results. 
Measurements of the force of gravity relative to the base 
station, Washington, were made at twenty six stations, the 
fieldwork occupying 150 days. The famous series carried 
out in India in connection with the Great Trigonometrical 
Survey, comprising thirty one stations, required six years for 
its completion. Some allowance should, however, be made 
for the poor transportation facilities existing there. It is 
true that work more rapid than that of the past season, has 
been performed in Europe, but this has been done only by 
using approximate methods in rating chronometers and in 
comparing the pendulums therewith. Of the present series 
as planned by Dr. Mendenhall, eighteen stations follow at 
fairly systematic intervals the course of the transcontinental 
triangulation along the thirtyninth parallel as far west as 
Salt Lake City; the others are located further north, in the 
eastern and central part of the country, with three stations 
in the Yellowstone park. The series along the thirtyninth 
parallel, on account of the wide variety of orographic feat¬ 
ures traversed so nearly in the same latitude, is peculiarly 
well adapted to throw light on important questions, such as 
