896 Report of the Great Trigonometrical Survey. [No. 4 



Survey Offices, all over India, in order to reduce them to a common 

 datum. As a first step towards this desirable measure, I have pub- 

 lished a volume of Tables of Levels, based on the Spirit-Levellino- 

 Operations of this Survey, and reduced to the mean sea level <rf 

 Karachi Harbour, as their datum. Additional volumes will be pub- 

 lished as soon as possible. They will enable officers of the Public 

 Works and Eailway Departments to reduce their levels to the mean 

 sea, by connecting them with the nearest Bench Mark, or Station of 

 the Trigonometrical Survey. In most instances, however, the business 

 of connecting will probably devolve on the Survey Department. At 

 present, we have only one Levelling Party, which is employed in 

 Bengal ; I therefore submitted a project for the formation of other 

 parties to carry on operations, simultaneously, in the Madras and 

 Bombay Presidencies, as the only means of speedily accomplishing an 

 operation, of which the practical value will be greatly enhanced by 

 early completion. Unfortunately, financial reasons have interfered to 

 prevent this proposal from being sanctioned. 



I now proceed to report on the Astronomical Observations for the 

 determination of the Latitude and Longitude of the Andaman Islands, 

 which were instituted on a representation by the Superintendent of 

 Port Blair, that the erroneous positions assigned to some of these 

 Islands, in the published Charts, endangered the safety of ships sailing 

 between Calcutta and Singapore. Under the orders of Government, in 

 the Home Department, the Surveyor General had deputed a Surveyor, 

 Mr. Mcolson, to conduct the necessary observations, the superinten- 

 dence of which was subsequently transferred to the Trigonometrical 

 branch of the Survey. 



Mr. Mcolson started from Calcutta early in December, 1861, to 

 reconnoitre the Coco and Andaman Islands. He found that, in order 

 to take a complete Series of Astronomical Observations at the Great 

 Coco, it would be necessary to have a steamer pbced at his disposal 

 for some weeks, to keep up his communication with Port Blair, and 

 bring the necessary supplies for his party. 



About this time, a communication was received from the Bombay 

 Government, representing that there was as much doubt about the 

 accuracy of the position of Port Blair, as of that of the Coco Islands. 

 Under these circumstances, it seemed advisable that Mr. Mcolson 

 should begin operations by fixing Port Blair, in order that the proposed 



