MERIDIAN ON THE COAST OF COROMANDEL. 155 



on the limb of the instruinent, 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 in a Contrary direction to what it had been 

 moved before ; which latter method has been re- 

 commended by the maker. This circumstance re- 

 specting the axis should be most scrupulously at- 

 tended to before the adjustment of the micrometers 

 begin, 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 there- 

 fore one division to one second, tlie circle caa 

 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 necessary to sink the axis. The 

 carriage being performed altogether by men, there 

 is not that jolting which any other mode of con- 

 veyance is subject to, and as I found, that a con- 

 siderable time was taken up in adjusting the axis 

 before the revolutions of the micrometers could be 

 brought to their intended limits, I therefore laid 

 it aside, unless under the circumstances above 

 mentioned. 



The semicircle of the transit telescope is gradu- 

 ated to 10' of a degree in place of 30', which was 

 the case with the sem.icircle described by General 

 Roy, and the m'.crometer to the horizontal micro- 

 scope applied to this semicircle, making one revo- 

 lution in two minutes, and five revolutions for ten 

 minutes on the limb; and the scale of the micro- 



