1868.] Proceedings of the Asiatic Society. 93 



For the longitudes it was decided to adopt the methods of Moon 

 Culminations and Lunar Zenith distances, employing, for all the ob- 

 servations, one of the large Astronomical Circles which were brought 

 out to India by Colonel Everest, and are described in his account of 

 the Indian Arc ; their vertical circles have a diameter of 3 feet, and 

 the telescopes a focal length of 4J feet and a magnifying power of 

 about 80. A temporary observatory with rotating dome was con- 

 structed at Calcutta and sent to Port Blair. Mr. Nicolson, an As- 

 sistant to the Surveyor General, was deputed to take the observations, 

 and as from his previous training in the Trigonometrical Survey he 

 was well qualified to observe Transits and Zenith distances, and as 

 the latter observations can be multiplied to any desirable extent, 

 whereas but few occupations and culminations can be observed during 

 a short time, he was directed to base his operations on Lunar Zenith 

 distances. He was furnished with an astronomical clock, a mean 

 time chronometer, a collimater, a barometer, and thermometers. 



His residence at Port Blair was protracted over a far longer period 

 than had been anticipated ; it was hoped that he would have been able 

 to complete his observations before the commencement of the rainy 

 season of 1862, but what with delays in getting a vessel to transport 

 him and his instruments to Port Blair, delays in the voyage, and dif- 

 ficulties in getting workmen to setup the observatory, his preparations 

 were only completed just before the monsoon set in, and for several 

 mouths the weather prevented any continuous observations. Thus 

 the work has been spread over a long time ; but the results should be 

 improved thereby, as the tabular errors of the moon's place are more 

 likely to vary and tend to cancel each other in a long than in a short 

 period. 



After the greater portion of the observations had been completed, 

 an accident happened to the astronomical clock which rendered it 

 useless for a time ; the chronometer was therefore employed for the 

 remaining observations. Time was determined each night by the me- 

 ridional transits of at least four Nautical Almanac Stars, half of which 

 were observed with the illuminated pivot of the transit axis point- 

 ing to the east, and the remainder with it pointing to the west. The 

 lunar zenith distances were, as a rule, taken in pairs, with the illumina- 

 ted pivot to the left for one observation, and to the right for the 



