622 



geodist for the probability of a future correction being needed : as 

 to Captain Jacob's performance, I may distinctly state that I place 

 the greatest confidence in its accuracy, for he was furnished with an 

 excellent altitude and azimuth instrument by Dollond, which had a 

 good vertical circle, and he was not only a highly-talented mathema- 

 tician, but a most careful and skilful observer. 



Since I left India, in December 1843, the trigonometrical opera- 

 tions have abutted on the sea-coast at two other places, viz. at Cal- 

 cutta, by the completion of my north-east longitudinal series, and at 

 Karachi, by Colonel Waugh's western longitudinal series ; so that 

 now there are several additional data relative to the numerical value 

 of h, which it becomes necessary to take into consideration in the 

 determination of the proper corrections to be applied to the mea- 

 sured bases, and the lengths of my two terrestrial arcs, as given in 

 synopses B and A. 



I have for some time been in correspondence with Colonel Waugh 

 on this subject, and in due time hope to be able to communicate 

 what final conclusion we arrived at ; but, in the meanwhile, as it 

 may be interesting to the Fellows of the Royal Society to know the 

 provisional state of the question, I here subjoin some extracts of his 

 last received communication. 



"By the completion of the Calcutta meridional series, as well as 

 the north-east longitudinal series, we obtain a continued chain of 

 triangulation, extending from the sea-level near Calcutta along the 

 Calcutta meridian to the Sonakoda base, thence along the north- 

 east longitudinal series to Dehra Dun, thence down the great arc, 

 and along the Bombay longitudinal series to the sea-level at Bombay. 

 The result of trigonometrical levelling along this course of 2127 

 miles, comprising 1171 miles of hills, and 956 miles of plains, be- 

 comes verified ; the discrepancy in height being 6' 76 feet or 0*003 

 foot per mile. 



" By the completion of the great longitudinal series, we get another 

 continuous chain of triangulation from the sea-level near Calcutta 

 to the same level at Karachi. This chain embraces the Calcutta 

 meridional series, the north-east longitudinal series, the northern 

 section of the great arc from Dehra Dun to Sironj, and the great 

 longitudinal series from Sironj to Karachi. This series is 2082 

 miles in length, comprising 1041 miles of plains, and 1041 miles of 



