212 



GEODETIC OBSERVATIONS. 



in 1802-5 to determine the length of a degree of longitude, so the 

 comparison of determinations by ancient and modern methods 

 became peculiarly valuable. 



The procedure adopted in the telegraphic operations was as 

 follows : — Two observers were employed, one for each of the two 

 stations to be connected. Each observer furnished with a complete 

 set of instruments took up his position and made the most accurate 

 determination possible of the error of his own clock, and at certain 

 times both observers working in concert determined, by means 

 of electric signals transmitted between the stations, the absolute 

 difference between the clocks at a given instant. With these data 

 the difference of longitude between the stations or arc of longitude 

 becomes known.* 



The results of the first field season's operations were unfortunately 

 unsatisfactory : one of the transit instruments showed discordances 

 in the ' constant for collimation ' which were eventually traced to a 

 fault in the joint of a telescope, so the observations were rejected 

 and re-measurement was necessary. 



During 1875-76 the following arcs were measured by Captains 

 Campbell and Heaviside : — 



Haidarabad (Bolarum) — Bombay. Madras — Haidarabad (Bolarura). 

 Bellary— Bombay . Madras — Bellary. 



Haidarabad (Bolarum) — Bellary. Bangalore — Bellary. 



During the next year (1876-77) the following arcs were measured 

 by the same officers : — 



Vizagapatam — Madras. 



V i /. a gapatam — Bell a it . 

 Mangalore — Bombay. 

 In almost all the cases the trigonometrical values were found to be 

 greater than the telegraphic. This is due partly to the circumstance 

 that the constants for the figure of the earth, used in the computa- 

 tions of the geodetic latitudes and longitudes of the Indian Survey, 

 are not quite exact, and partly to local deflections of the plumb-line 

 at the >tatioiis of observations, which indicate (in accordance, it may 

 be observed, with the results of Captain Basevi's pendulum observa- 

 tions) a probable greater density in the strata of the earth's crust 

 under the beds of oceans than under continents. 



* Volume IX.. of the operations of the Great Trigonometrical Survey of India, 

 page xiv. 



