44 MEASURE OF A DEfiHEE ON THE COROMANDEL COAST. 



jng the errour of the semicircle, and to ascertain the place 

 of the fixed wire in the horizontal microscope, so as to divide 

 the errour. It has appeared in the event, that the telescope 

 being in its right position, (that is, when the limb and mi- 

 croscope were on the left hand,) and the fixed wire placed 

 at zero on the semicircle, when the circle or iimb of the 

 theodolite was turned 180° in azimuth, and the telescope 

 turned over, the fixed wire was then distant from zero on 

 the opposite part of the are by a mean of a great many ob- 

 servations 2' 57", the half of which is therefore the errour. 

 This half was carefully set off from zero by the movable 

 tnicrometer wire, and the fixed one brought to coincide with 

 if. On the right application of this errour, there will be 

 1' 28"*5 to add to the elevations and subtract from the de- 

 pressions. The observations for determining this quantity 

 were repeated at different times, and under the most favour- 

 able circumstances; the adjustments of the whole instru- 

 ment being frequently examined, and the level applied to 

 the telescope reversed at most of the observations. For the 

 Lin«of colli- line of collimation, as these corrections depend on having a 

 jridtion. ^-eij defined object, 1 fixed a bamboo upwards of a mile dis- 



tant froin the observatory tent, and tied round it sevoril 

 narrow stripes of black silk, one of which was near the ho- 

 rizontal wire when the axis of the telescope intersected the 

 staff after being brought to a level by the bubble. Then 

 the instrument being adjusted, and the telescope directed to 

 the bamboo being perfectly level, and the wire of the mi- 

 crometer in the piece brought to the intersection of the cross 

 wires, the angular distance to the mark on the bamboo was 

 measured by the rur^s pf 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 

 reversed or put again into the same Ys. The levelling ad-: 

 justment was then made, and the angular distance froip the 

 intersection of the wires to the black mark again taken, half 

 the difference between which and the former was of course 

 the errour of collimation. This errour 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 ryds. After that, the remaining er- 

 rour 



