GEODETIC OBSEKVATIONS. 213 



On the completion of the operations between Bombay and Manga- 

 lore, Captains Campbell and Heaviside proceeded to determine the 

 differences of longitnde between Bombay, Aden, and Suez, in order 

 to complete the connexion between England and India, of which the 

 section from Greenwich to Suez had been executed on the occasion 

 of the Transit of Venus in 1874, under instructions from Sir George 

 Airy, the Astronomer Royal. The Eastern Telegraph Company 

 kindly granted the gratuitous use of their cables, not only during the 

 actual work of signalling, but also during the preliminary tentative 

 measures. In these operations Captain Campbell devised a sort of 

 automatic method of comparing the clocks through the cables, so 

 that no personal errors of observing or repeating signals could 

 enter. The results were to establish the following values* : — 



h. m. s. 



Station at Aden - - = 2 59 55/83 ~] 



Observatory at Bombay - = 4 51 15 - S1 > p ' ..^ 



Observatory at Madras - = 5 20 59 -34 J 

 The longitudes of all places in India are usually referred to Green- 

 wich through the Madras observatory, the position of which has 

 been determined at various times by astronomical observations. The 

 latest determination had been 5h. 20m. 57-3s., or 80° 14' 195' 

 east of Greenwich, and this had been for many years the accepted 

 value, being that given in the Nautical Almanac. The effect of 

 these electro-telegraphic observations was to show that this value 

 was about 2 seconds of time or 30 - 57 seconds of arc in deficit of 

 what is probably the true value, i.e., 80° 14' 50-03" E.* 



In 1877 both officers took furlough, and on their return the 

 Afghan war intervened, but in 1880-81 operations were resumed 

 and the following geodetic arcs were measured : — 



Bombay — Disa. ] Jabalpur — Haidarabad (Bolarum). 



Disa — Karachi. I Jabalpur — Agra, 



Bombay — Karachi. | Jabalpur — Disa. 



Jabalpur — Bombay. j Agra — Disa. 

 During the following season seven arcs were measured between 

 Faizabad, Agra, Jabalpur, Hazaribagh, Calcutta, and Jalpaiguri, 



* See p. xviii. of Vol. IX. of " Account of Operations of the Great Trigonometrical 

 " Survey of India." It has been recently suggested that Indian longitudes should be 

 referred direct to Greenwich, so as to avoid the discrepancies arising from the fact of the 

 true longitude of Madras Observatory being above 2^ minutes west of its accepted 

 position on Indian maps. 



