1865.] Pendulum Base Observations for India. 425 



" An Account of the Base Observations made at the Kew Observa- 

 tory with the Pendulums to be used in the Indian Trigonome- 

 trical Survey." By BALFOUR STEWART, M.A., LLJX, F.R.S., 

 Superintendent of the Kew Observatory, and BENJAMIN LOEWY, 

 Esq. Received June 13, read June 15, 1865. 



Her Majesty's Indian Government, on the recommendation of the Royal 

 Society, lately decided that pendulum observations shall be made at different 

 stations in India in connexion with the Great Trigonometrical Survey of 

 that country. 



The object of these proposed observations may be stated in a very few 

 words. The labours of those engaged in the Trigonometrical Survey have 

 already disclosed the fact that the direction of the plumb-line in the north- 

 ern stations of India was influenced to some extent by the mass of the 

 Himalayas, and it was therefore thought highly desirable that the influ- 

 ence of these mountains upon the intensity of terrestrial gravity should be 

 investigated in addition to their influence upon its direction. The propriety 

 of this view will at once be evident, if we reflect that by knowing the 

 change produced not only upon the direction of gravity, but also on its 

 intensity, we know at once all the particulars of the disturbing mountain- 

 force both as regards magnitude and direction. 



It was therefore with the view of ascertaining the alteration which these 

 mountains might cause upon the intensity of gravity that the Indian pen- 

 dulum observations were decided upon. In consequence of this decision, 

 Captain Basevi, R.E., and first assistant in the Survey department, was ap- 

 pointed to superintend the observations, and instructed to repair to the 

 Kew Observatory previously to his departure for India, in order to become 

 acquainted with the necessary instruments, their adjustment, and the 

 method of observing with them. 



After attending daily at the Observatory from the beginning of Septem- 

 ber to the middle of November, this officer was perfectly instructed in 

 every particular necessary for the practical part of these observations, as 

 well as for their calculation and reduction. He was then obliged to leave 

 for India, being prevented by his early departure from making the necessary 

 base determinations with the instruments at Kew Observatory. 



The best arrangement of apparatus formed the subject of careful discus- 

 sion with Colonel Walker, Superintendent of the Indian Survey ; and the 

 experimental arrangements ultimately adopted received the sanction of this 

 officer, who, besides suggesting several improvements, made himself tho- 

 roughly acquainted with all the details of the apparatus. 



VOL. XII. 2 K 



