MERIDIAN ON THE COAST OF COROMANDEL. 157 



mark on the bamboo was measured by the runs of 

 that micrometer, and the wire brought back to 

 the point of intersection of the other wires. The 

 circle was then turned half round and the telescope 

 reserved or put again into the same Ys. The level- 

 ling adjustment was then made, and the angular 

 distance from the intersection of the wires to the 

 black mark again taken, half the difference be- 

 tween which and the former was of course the er- 

 ror of coliimation. -This error was repeatedly re- 

 duced till it became very small, half by the finger 

 screw of the clamp to the semicircle, and half by 

 the adjusting screws to the levelling rods. After 

 that, the remaining error w'as repeatedly examined 

 and found to be 2", 36 to be subtracted from the 

 elevations and added to the depressions when the 

 telescope is in the ordinary position, or w^hen the 

 semicircle and microscope are on the left hand ; 

 but vice vei'sa when in the contrary position. These 

 errors of the semicircle and line of coliimation being 

 opposite, the result from comparison will be, "That 

 when ekvations or depressions are taken with the se- 

 micircle, V26" must be added to t\\Q former, and 

 subtracted from the latter.'' 



And that when the elevations and depressions 

 are taken by the micrometer in tlie eye piece 2",3G 

 must be deducted from the elevations and added to 

 the depressions. 



The micrometer in the focus of the eye-glass of 

 the transit telescope is the same in all respects as 

 the one mentioned by General Roy, that is to say, 

 the circle or scale is divided into one hundred divi- 

 sions, and there is a nonius fixed to the upper part 

 of the telescope, which defines the revolutions of 

 the microm.eter as far as ten for the elevations and 

 ten for the depressions. The following experiments 



