I 
GRAVITY MEASUREMENTS. 33 
the continental variation of gravity, the condition of the 
earth’s crust, and the proper method of reduction to sea 
level. This line of stations, commencing at the Atlantic 
coast, ascends to near the crest of the Appalachians, trav¬ 
erses the great central plain, gradually increasing in altitude 
from 495 to 6,041 feet (151 to 1,841 meters); then rises to 
the high elevations of the main chain of the Rocky moun¬ 
tains, reaching an altitude of 14,085 feet (4,293 meters) at 
Pikes Peak; descends into the eroded valleys of the Grand 
and Green rivers, crosses the summit of the Wasatch ridge, 
and finally descends to the great western plateau of the con¬ 
tinent. 
The half second pendulum apparatus used in this work 
was exhibited before this Society in February, 1891, and is 
described in the Report of the Coast and Geodetic Survey 
for 1891, pages 503 to 564, so that only a brief description is 
needed here. The complete outfit comprised a set of three 
quarter meter invariable pendulums, an air tight case in 
which they were swung, a dummy or temperature pendu¬ 
lum, flash apparatus, light air pump, dry cells and various 
other accessories, a portable astronomical transit suitable for 
either time or latitude observations, a small chronograph, 
and an astronomical tent. Including packing cases, the 
whole weighed about 700 pounds (318 kilograms). The 
pendulums are of the new type, with agate knife-edge and 
plane inverted from the usual position, following the idea 
of Dr. Mendenhall. The case is a heavy metal one, provided 
with leveling screws, windows, arrangements for starting, 
stopping, raising, and lowering the pendulums from the out¬ 
side, and is air tight when the lid is properly placed. There 
is a microscope for reading the arc of oscillation, and a small 
pendulum with level tube in its head for making the knife- 
edge horizontal. The temperature is obtained from a ther¬ 
mometer whose bulb is inserted in the stem of a dummy 
pendulum, which is of the same size and metal as the swing¬ 
ing one and is held in the case near it. The pressure is read 
by a small manometer hung in the case, the air being ex- 
