84 Differences of Meridian at Madras, [No. 97. 



These were the best telescopes, and used by the steadiest 

 observers, but not always the same instrument, by the same 

 person. The mean of these, with that of my individual sights, 

 give 2m. 31s. '40 for the difference, which must be near the 

 truth, and for my station 5h. 18m. 32s. -37, or 79° 38', that 

 is, if the meridian of Madras be correctly settled. 



I may further mention, that I took and compared with the 

 Nautical Almanac six immersions, and an equal number of 

 emersions of the 1st satellite — they give, h. m. s. 



6 immersions, 5 18 32 *7 



6 emersions, 5 18 38 *3 



Mean, . . 5 18 35 5 



All taken with Troughton's 46-inch telescope, power 64, and 

 by the same person. 



I have extended these remarks to a far greater length 

 than I intended, but perhaps some notice of another mode of 

 investigating the longitude, may be useful to the officers 

 of the Bengal army, who are serving with our regiments 

 from Afghanistan to China. This is the well known mode 

 of lunar distances from the sun and stars, which has not 

 been so much used on land, as it might be, and with 

 very great advantage, in the clear atmosphere of Asia; fre- 

 quent opportunities of seeing the moon and stars and sun 

 occur ; the mode of operating is not difficult, and the instru- 

 ments required are easy of carriage, and do not require any 

 fixed supports. The calculation is rendered simple, and the 

 results satisfactory, by means of the correct data in the 

 Nautical Almanac ; with these, and that most excellent of all 

 instruments, Troughtotfs reflecting circle, any officer may, with 

 a little practice, do good service to geography. I wish it to 

 be understood, that it is not by the sextant that we are to 

 look for such results, it being only a second best instrument, 

 but from the circle, which is, though a little heavier, equally, 

 nay, more convenient, in use, than that imperfect part of 

 a circle, the sextant; which should never be used on land, 

 nor at sea either, if satisfactory longitudes are hoped for ; and 

 where are they more required ? 



