GEODETIC OBSERVATIONS. 211 



the variations of the force of gravity at sundry stations of the 

 Survey, situated on mountains, table lands, the interior of the 

 continent and the coast lines, which was commenced in the year 

 1865 and is described in the fifth volume of the Account of the 

 operations of the Great Trigonometrical Survey of India. And 

 when the electric telegraph had been introduced into India, the 

 advantage was taken to commence the measurement of differential 

 longitudes between certain stations of the Survey, co-ordinating 

 with the differential latitude measurements and thus to contribute 

 to the science of geodesy determinations of a number of longitudinal 

 arcs which were to supplement and be combined with the time- 

 honoured Meridional Arcs. 



One of the first preliminary steps for the electric determination 

 of differential longitudes was to procure the necessary apparatus, 

 the astronomical clocks, transit instruments, chrouograph and 

 electric appliances. On the recommendation of Colonel Walker, 

 then Superintendent of the Great Trigonometrical Survey, the 

 expenditure for this was sanctioned by the Secretary of State, and 

 Colonel Strange, a retired officer of the Survey, residing in London, 

 was entrusted with the task of designing and superintending the 

 construction of the instruments. He had already acquired a high 

 reputation for skill in mechanism generally, especially in regard 

 to delicate geodetic instruments. Messrs. Frodsham, of London, 

 were selected for the task of making the clocks, Messrs. Cooke and 

 Sons, of York, for the transit instruments, and Messrs. Bichens and 

 Hardy, of Paris, for the chronographs and electric apparatus. 

 Before being sent out to India the instruments were examined 

 at the observatory attached to the India Stores Depot, by Colonel 

 Strange, assisted by Captain W. M. Campbell, who subsequently 

 took a prominent share in the observations themselves. 



The instruments were received in India in 1872, and were placed 

 in the hands of Captains Herschel and Campbell, who were then at 

 Bangalore, which, being connected both by railway and telegraph 

 with Madras, was a convenient starting point for the operations. 

 The arc Madras — Bangalore was first' measured, and then the sister 

 arc Bangalore — Mangalore. The total arc was a very interesting 

 one. In length it was not so great — its amplitude was 5° 24' 12", 

 and lineal measurement 364 miles — but it was situated much nearer 

 the Equator than any other measured arc, and, moreover, it was 

 on that very arc of parallel that Colonel Lambton had endeavoured 

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