MEASURE OF A DEGREE ON THE COROMANDEL COAST. 43 



scopes, and in an adjustment to the vertical axis, by which 

 the circle can be moved up or let down by means of two 

 capstan screws at the top of the axis. These are mentioned 

 in the Philosophical Transactions for 1795, in the account 

 of the trigonometrical survey. By sinking the circle on the 

 axis, it is better adapted for travelling, and when the micro- 

 scopes are once adjusted to minutes and seconds, on the 

 limb of the instrument, the circle can always be brought 

 back to the proper distance from them. Great attention 

 however is necessary in bringing the axis down, so that the 

 wires in each microscope being fixed at opposite dots on the 

 limb, they may coincide with the same dots when the circle 

 is turned half round, or made to move entirely round, and 

 jn a contrary direction to what it had been moved before ; 

 \vhich latter raethodhas been recommended by tiie maker. 

 This circumstance respecting the axis should be most scru- 

 pulously attended to before the adjustment of the microme- 

 ters begins, so that when by arranging the lenses in such a 

 manner that ten revolutions of the micrometer may answer 

 to ten minutes on the limb, and therefore one division to 

 one second, the circle can always be brought to its proper 

 height, by trying the revolutions of the micrometer. 



It has however been found from experience, that unless 

 in cases of very long and troublesome marches, it is not ne- 

 cessary to sink the axis. The carriage being performed al- 

 together by men, there is not that jolting which any other 

 mode of conveyance is subject to, and as I found, that a, 

 considerable time was taken up in adjusting the axis before 

 the revolutions of the micrometers could be brought to thdr 

 intended limits, 1 therefore laid it aside, unless under the 

 circumstances above mentioned. 



The semicircle of the transit telescope is graduated to 10' Semicircle of 



of a degree in place of 30', which was the case with the l"'^^ ^'^^'^^^ ^^ 



, . ^ ^ . lescope. 



semicircle described by General Roy, and the micrometer 



to the horizontal microscope applied to this semicircle, 

 making one revolution in two minutes, and five revolutions 

 for ten minutes on the limb ; .and the scale of the microme- 

 ter being divided into sixty parts, each part is therefore two 

 seconds of the circle. 



A number of experiments have been made for determin- Ei rour of the 



•^^ semicircle. 



