82 Colonel A. R. Clarke on tjie Figure of the Earth. 



Colonel Walker's last Report contains the details of eleven 

 determinations of'difference of longitude by electro-telegraphy, 

 with the corresponding geodetic differences. The differences 

 of longitude are between Mangalore and Bombay on the west 

 coast, Vizagapatam and Madras on the east coast, and Hydra- 

 bad, Bangalore, and Bellary in the interior. Bellary holds a 

 somewhat central position in the polygon formed by the other 

 points ; Mangalore and Madras are very nearly in the same 

 latitude ; and Bangalore is midway between them. These dif- 

 ferences of longitude have been determined with every refine- 

 ment of modern science, and, taking into account the uncer- 

 tainty of local attraction, may be considered little, if at all, 

 inferior to latitude-determinations. "When the operations 

 were commenced," says Colonel Walker, " I determined that 

 they should be carried on with great caution and in such a 

 manner as to be self-verificatory, in order that some more satis- 

 factory estimate might be formed of the magnitudes of the 

 errors to which they are liable than would be afforded by the 



theoretical probable errors of the observations The 



simplest arrangement appeared to be to select three trigono- 

 metrical stations A, B, C, at nearly equal distances apart on a 

 telegraphic line forming a circuit, and, after having measured 

 the longitudinal arcs corresponding to A B and B C, to mea- 

 sure A C independently as a check on the other two arcs." 



The eleven determinations of difference of longitude between 

 the seven points named above give thus five equations of con- 

 dition among themselves, which enable us to assign a system 

 of minimum corrections to the several determinations. The 

 following Table contains the observed differences, together 

 with the computed corrections, half-weight being given to the 

 first two determinations, which were the earliest made, and 

 which are affected Avith some slight defects, then undiscovered, 

 in one of the transit-instruments :— 







Electro-telegraphic 





Year. 



Arc. 



difference of lon- 



Corrections. 







gitude. 





1872-73. 



Madras — Bangalore. 



J 39 45-63 



+2-010 



„ 



Bangalore — Mangalore. 



2 44 11-54 



+ 1-690 



1875-76. 



Hydrabad — Bombay. 



5 42 12-74 



-0-452 



„ 



Bellary— Bombay. 



4 6 44-39 



-0-393 



j> 



Hydrabad — Bellary. 



1 35 28-25 



+0040 



„ 



Madras — Hydrabad. 



1 43 40-38 



-0-412 



5J 



Madras — Bellary. 



3 19 8-45 



-0-192 



,, 



Bangalore — Bellary. 



39 20-46 



+0-160 



1876-77. 



Vizagapatam — Madras. 



3 2 26-78 



+0-401 



?> 



Vizagapatam — Bellary. 



6 21 35-84 



-0401 



;> 



Mangalore — Bombay. 



2 1 5054 



+0-845 



