VIII. On the Intensity and Direction of the Force of Grarity in India. 



By Lieut.-Colond S. G. BCJRUABD, H.E., F.R.K. 



Received March 30, Read April 13, 1905. 



['PLATES 14-20.] 



(1.) The Pendulum Observations of /.sv/,-7-7'.^. 



BETWEEN 1865 and 1873 observations were taken at 31 stations in India by 

 Captains BASEVI and HEAVISIDE with the Royal Society's seconds pendulums. 

 The results were published in Vol. V. of the ' Account of the Operations of the 

 Great Trigonometrical Survey of India,' and have been subsequently discussed by 

 many authorities.* 



Captain BASEVI expressed his results in terms of N, the number of vibrations of 

 the mean pendulum observed in a mean solar day. The International Geodetic 

 Association show their results in dynes, and it is desirable that we should follow their 

 example. We have, therefore, to change the notation employed by our predecessors. 



The fundamental formula, expressing the relation between the length of a 

 pendulum, its time of vibration and the accelerating force g, is t = TT \/(l/g)- If N be 

 the number of vibrations, which a pendulum of length / makes in a mean solar day of 

 86,400 mean time seconds, then 



M _ 86400 _ 86400 /g 



T~ ~ \f 1 ' 



v 77" r ' 



where t is the time of vibration. 



If N becomes N + c/N, when g becomes g + dg, then 



> 



By this formula, if certain values of N and g be adopted for a Standard Station, 

 the results of the older pendulum observations can be converted, and the symbol g 

 substituted for N.t 



The pendulum observations in India were undertaken, and are now being extended, 

 with the object of determining the difference between the force of gravity as observed 



* See 'Phil. Trans.,' A, vol. 186, 1895; HELMERT'S 'Die Schwerkraft im Hochgebirge ' ; HELMERT'S 

 'Hdhere Geodiisie'; CLARKE'S 'Geodesy'; FISHER'S 'Physics of the Earth's Crust.' 

 t dg = 0'0226(/N is a rough rule, sufficiently accurate for many purposes. 

 VOL. CCV. A 394. 2 P 12.10.05 



