THE RELATIVE FORCE OP GRAVITY AT KEW AND GREENWICH 
555 
swings in India ; and this has also been done to the revisionaiy swings at Kew and 
Greenwich, to produce the vibration-numbers which have already been set forth. 
But, in the course of the operations in India, Captain Basevi reinvestigated both 
the temperature and the pressure corrections of his two pendulums, those of No. 4 
with great elaboration. A series of several sets of swings was made with it at each 
of the successive pressures of 0’6, 1’9, 4’2, lO'O, 17'5, and 27'7 inches, at the tempera¬ 
ture of about 101° F; another series at the same pressures, at the temperature of 
about 53°; and a third at the pressures of 1’9 and 4 "2 inches and temperature of 
a,bout 80°. He came to the conclusion that the pressure correction is best represented 
by an empirical formula of three terms, 
A/3^ -f B;8 + 
in which the second term is the correction for buoyancy. Then he assumed A to 
be = x \/461° -b ^ and C = y -f- \/461° + ^ — 4G1° being the absolute xero of the air 
thermometer—and formed a corresponding series of equations for the determination of 
X and y from his fourteen sets of observations. The solution of these equations gave 
X = '022 d: '002, and y — T23 ’025, which values satisfied the equations of 
condition very satisfactorily. 
But the subject is one of great complexity and difficulty, as will be seen in 
consulting Chapter VI. of the Indian pendidum volume. Something appears to l)e 
wanting to explain the inconsistencies between vibration-numbers deiived from 
different series of very accordant observations which are occasionally met with. 
Possibly it may be necessary to take cognisance of the atmosplieric humidity during 
the observations, which has never been done hitherto. Or it may be tliat the incon¬ 
sistencies arise from changes in the relative conditions of the bearing surfaces of the 
knife-edge and the agate planes, which are met witli on successive transpositions of 
the pendulum, and which the observer cannot control. 
Transposition is almost invariably attended with a change in the vibration-number; 
but in the Indian operations it was found that the changes were not constant in 
either sign or magnitude ; it is shown, at page 114 of the volume adready cited, that, for 
the whole of the 34 stations of observation, the mean value of M — P ranges from 
+ '54 to — ’52, and has an average value of + '07 '93 for Pendulum No. 4, and ranges 
from -b ’67 to — '59, with the average value — '04 '03 for Pendulum No. 6 (1821). 
In reducing the Indian swings for investigating the pressure correction. Captain 
Basevi’s observations of the vibration-numbers at different pressures were employed 
directly, without having recourse to his empirical formula. The observed vibration- 
numbers at each pressure were reduced to a vacuum b}^ the Kew formula with the 
numerical constant '32, and then the results for the higher pressures were compared 
with the result for the lowest pressure, which, being O'G of an inch, was very close to 
the vacuum; and it was found that the higher piessures required residual positive 
corrections, increasing with the temperature as well as tlie pressure; at the highest 
4 B 2 
