426 



Pendulum Base Observations for India. 



[1865. 



A room suitable for these observations was constructed in the south-east 

 corner of the Observatory, the expenses of which were defrayed from the 

 Government Grant Fund of the Royal Society. 



The following simple diagram will be sufficient to show the experimental 

 arrangement in the Pendulum-room. 



C is the place for the clock, which is connected with the transit-instrument; P is the 

 pillar bearing a slab attached also to the wall at W, to which the receiver (E) is 

 rigidly fixed. 



T is the telescope for the observations of the coincidences, mounted on a pillar which 

 stands in a depression, so that the observer is not under the necessity of kneeling 

 down during the observation. 



In every other part the arrangement is entirely similar to that de- 

 scribed by General Sabine in the Philosophical Transactions for 1829, with 

 this difference, that the receiver was in our experiments a copper one with 

 glass windows. The whole of the apparatus was made by Mr. P. Adie, 

 who deserves the highest praise for the excellent manner in which the work 

 was executed by him. 



The pendulums used were those marked No. 1821 and No. 4, used 

 formerly by General Sabine in different parts of the globe. The former 

 was also used by Mr. Airy in his Harton experiments. 



Method of registering and reducing the Observations. 



The manner in which the number of vibrations, made by a detached 

 pendulum, are determined from a series of observed coincidences with the 

 pendulum of a clock has been so often described, that we may refer to the 

 writings of Kater, Sabine, Baily, and others on the subject. The esta- 

 blished methods have been followed throughout in these experiments, and 



