MEASURE OF A DEGREE ON THE COROMANDEL COAST. 4&' 



turned half round ; the adjustment examined and corrected," 

 if necessary. This being done, the degrees and minutes, 

 &c., on the arc were noted down, as was also the particular 

 division on the micrometer scale, at which the index stood, 

 and the fractional part of a division in case there was any. 

 In this state every thing remained to within fifteen or twenty 

 minutes of the time the star was to pass, when I repaired to 

 the tent, and again examined whether the wire bisected the 

 dot; if it did not, the instrument was again adjusted to the 

 same dot, and the horizontal axis also examined by the up- 

 per microscope, all this being done, the sector was placed 

 in the meridian. 



When the star entered the field of view, the micrometer Observation oi 

 1 1 1, 1 11- 1 • the star.. . 



was moved gently till the star was near the horizontal wire, 



but not bisected till it came near the vertical, that the mi- 

 crometer might not be turned back, but continue moving in 

 the same direction. This I did to avoid any false motion in Caution re- 



the micrometer screw, and I was led to this precaution by sp.®^''"§ ^^^ 



. ^ •' micrometer. 



the repeated experiments I had made in examining the divi- 

 sions on the arc, for it sometimes happened after moving the 

 arc oVer one of the divisions till the wire bisected the next 

 dot ; and then turning it back again, that the index of the 

 micrometer was not at the same second, but had passed over 

 it perhaps one, and sometimes two seconds ; but by moving 

 over the next five minutes in the same direction, the number 

 of revolutions and seconds were always what they ought to 

 be, to some very small fraction. This anomaly, however, 

 only happened in some situations of the screw, and to avoid 

 any errours arising therefrom, I adopted the above method. 



The zenith distance of the star being now had, on one part Zenith dis- 

 of the arc or limb, after the same process had been gone bl)"h^pans"of'* 

 through the next night, with regard to the adjustment, the the arc 

 zenith distance was taken on the other part of the arc, by 

 turning the instrument half round on its vertical axis. The 

 mean of these two was therefore the true observed zenith dis- 

 tance, and half the difference v,'as the errour of coUiraation. 

 For applying these to the purpose in question, the mean of 

 the zenith distances being corrected for refraction, the decli- 

 nation of the star for each of these nights was corrected for 



Vol- X-X.—May, 1808. E nutation, 



