MiASHRE OF A DEfiREE ON THE COROMANDEL COAST* 47 



arc over l' 10" 08'", but. the scale being large, a small frac- 

 tion of a second can be easily defined. The construction, 

 and improvements to the zenith sector, are so well known, 

 that a minute description of it here would be unnecessary. 

 It will therefore suffice to saj^ that ai> far as so delicate an 

 instrument can be managed in a portable observatory, or 

 travelling tent, which never can offer the advantages of a 

 fixed, well contrived building, I have every reason to be sa- 

 tisfied with it. 



The time I commenced observing at Paudree station was Observation, 

 during the heavy part of the monsoon, which occasioned 

 frequent interruptions: and although I had intended ob- 

 serving by at least three fixed stars, I only succeeded to my 

 satisfaction in one, which was Aldebaran, With that star I 

 had a fortunate succession for about sixteen nights ; some 

 few of these observations, bein^ less favourable than the 

 others, were rejected. 



During the time I was at Trlvandeporum, near Cudda- 

 lore, the weather was settled and serene, and the nights 

 perfectly clear, so that I had an unlimited choice of stars, 

 but having been successful with Aldebaran, T chose that star 

 far determining the length of the arc. 



As I consider the celestial arc more likely to be erroneous Manner of ob- 

 than any terrestrial measurement, I have thought it neces- ad^justme^ Ac 

 sary to give some account of the manner of observing and instrument- 

 of adjusting the instrument, for, after two years experience, 

 I have found, that, notwithstanding the great powers of the 

 zenith sector, extreme delicacy and attention are requisite, 

 to render the observations satisfactory. Tlie following me- 

 thod of adjustment I have always practised. After having 

 brought the vertical axis nearly to its true position by the 

 adjusting screw at the bottom, or so that the wire of the W 



plummet would bisect the same dot when the telescope was 

 moved to the opposite side, or half round on the axis, I 

 then examined whether the dot at the centre of the hori- 

 zontal axis was bisected, and whether the wire moved in the 

 vertical plane clear of the axis; for unlets it be perfectly 

 free, all the observations will be false. When I had bi- 

 sected the dot, I either took out the microscope and looked 

 obliquely, or did the same by n aiKgnifying glass, and by 



that 



