1864.] Meport oftha Great Trigonometrical Survey. 397 



operations might be commenced at the place where the greatest faci- 

 lities for their execution existed. 



The inaccuracy of the present Charts of the islands lying between 

 Sumatra and Burma being admitted on all sides, it appeared necessary, 

 in the absence of any regular survey of those islands, to fix, by astro- 

 nomical observations, the positions of Acheen Head, Port Blair, the 

 Great Coco, or the Preparis Island, and an island in each of the other 

 groups, intermediate between Acheen Head and Cape Negrais. It is 

 believed that the relative positions of the mutually visible islands of 

 each group are already correctly shown on the Charts ; consequently, 

 by determining the absolute position of a point in each group, it would 

 be possible to rectify the existing Charts, without making a general 

 re-survey. 



Mr. Mcolson, having completed his reconnoissance, returned to 

 Calcutta in February, 1862, by which time one of the large 3-foot 

 astronomical circles of the Trigonometrical Survey had been got ready, 

 and a portable observatory, with rotating dome, constructed for the 

 observations. There was no good astronomical telescope available in 

 the stores of the Mathematical Instrument Department ; consequently, 

 Mr. Mcolson was directed to take all his observations, whether of 

 occupations, eclipses, or moon culminations, with the telescope of the 

 astronomical circle, which he could point to any part of the sky, 

 through the aperture in the rotating dome of the observatory. Owing, 

 however, to the small number of occultations and culminations which 

 occur monthly, and the risk of losing some of them in cloudy weather 

 Mr. Mcolson was directed to base his observations for Longitude 

 chiefly on the measurement of lunar zenith distances, for which the 

 astronomical circle is well adapted. He was supplied with an astro- 

 nomical clock, and all other necessary instruments, from the Calcutta 

 Observatory. 



In May, 1862, Mr. Mcolson had set up his observatory at Port 

 Blair, and was ready to commence observations. Unfortunately, the 

 season of fine weather had then nearly terminated ; the Monsoon set 

 in with unusual severity, nights favourable for observing were few and 

 far between, and, consequently, several months elapsed before the whole 

 of the necessary observations for Latitude and Longitude were complet- 

 ed. The work was further impeded by the delays attendant on postal 

 communication between Calcutta and Port Blair, making it very 



