1870.] 



On the Indian Pendulum-observations. 



99 



nexion with the operations of the Trigonometrical Survey of India, at the 

 recommendation of the President and Council of the Royal Society, are 

 now nearly completed, in conformity with the original programme of opera- 

 tions which was sanctioned by the Right Honourable the Secretary of State 

 for India, in his military letter No. 271, dated 23rd August 1864, to the 

 Governor- General in Council. 



(2.) The results are of much importance, not only as affording inde- 

 pendent information on the figure of the earth, but as throwing some light 

 on "the laws of the local variations of gravity which are superposed on 

 the grand variation from the poles to the equator ; " thus it will, I trust, 

 be conceded that they amply fulfil the purposes contemplated in the 

 ' Correspondence and Proceedings of the Council of the Royal Society 

 concerning Pendulum-Observations in India.' 



(3.) But, before the operations are brought to a close, I think it is 

 desirable that the President and Council of the Royal Society should be 

 informed of what has been done hitherto, and of what remains to be done 

 to carry out the original programme of operations ; also that they should 

 be invited to suggest any supplementary measures which they may consider 

 necessary in order to complete the operations, and thus perfect a work 

 which was commenced at the suggestion of the President and with the 

 hearty approval of the Council, and in the success of which they take a 

 lively interest. 



(4.) I have therefore prepared the accompanying note on the opera- 

 tions in explanation of what has been done hitherto, and of what remains 

 to be done to complete the original programme ; and I beg leave to request 

 chat the Secretary of State may be moved to communicate it to the 

 President and Council of the Royal Society, and to invite their opinions 

 and suggestions. The Note is accompanied with a map on which the 

 positions of the pendulum stations are indicated. 



I have the honour to be, Sir, 



Your most obedient Servant, 



J. T. Walker, Colonel R.E., 

 Supdt. Great Trigonometrical Survey of India. 



Note on the Pendulum-observations in India, which are being carried on 

 by Captain J. P. Basevi, in connexion with the operations of the Great 

 Trigonometrical Survey of India. 



The observations have been made with the two invariable pendulums 

 of the Royal Society, which are known as No. 4 and No. 1821 . The number 

 of vibrations in twenty-four hours is determined by observing the coinci- 

 dences of each pendulum with the pendulum of a clock by Shelton, which 

 is also the property of the Royal Society. The pendulums are swung, one 

 at a time, in the receiver of a vacuum apparatus out of which as much air 

 as possible is withdrawn by an air-pump, and the rate of the clock is de- 

 termined every night. 



