GRAVITY MEASUREMENTS. 
37 
swings cover the entire interval between time observations 
on succeeding nights ; or, if this is not done, it must be as¬ 
sumed that the rate of the chronometer during the few 
hours of observation is uniform with its daily rate, an as¬ 
sumption which experience has shown may be considerably 
in error. On the construction of an improved and quite 
portable air pump weighing but six pounds (2.7 kilograms), 
designed by Mr. Fischer, chief mechanician of the Coast and 
Geodetic Survey, it was found possible to reduce the air 
pressure in the receiver to less than one tenth of an atmos¬ 
phere in a few minutes, and experiment showed that at such 
a low pressure the length of swing could be extended to 
twentyfour hours if desired, though as a matter of conve¬ 
nience and to avoid very small arcs a length of eight hours 
was adopted.* An initial total arc of 55' was used, which 
falls off to about 20' in eight hours. A modification of the 
method of observing coincidences was also introduced. As 
a result of these smaller arcs the images were found to over¬ 
lap for several seconds, but by taking the mean of their first 
and last contacts great accuracy in observation could still 
be obtained even when the arc became as small as 5'. Each 
of these pendulums was swung twice—once direct and once 
reversed—the observations thus extending over fortyeight 
hours, so as to begin and end with time observations. If 
these were prevented by cloudy weather the swings were 
continued until time was obtained, but the very favorable 
conditions experienced during the past season rendered this 
necessary in only a few cases. Usually only three coinci¬ 
dences were observed at the beginning and at the end of 
each swing, these being sufficient to give without uncer¬ 
tainty the total number that had occurred. Two chronom¬ 
eters were used in the pendulum observations, coincidences 
being observed alternately with each. By a suitable switch 
*A plan of observation somewhat different from that here described 
was used on the first short trip, including the determinations at Cam¬ 
bridge, Boston, and Ithaca. 
