180 MEASUREMENT OF AN ARC ON THE. 



SECTION VII. 



Observations by the Zenith Sector for the latitude of 

 Paudree statioji, and the station near Trivandepo- 

 rum ; and the length of the celestial arc. 



The zenith sector, with which these observations 

 have been taken, was made by Mr. Ramsden, 

 and is the one aUuded to by General Roy, in the 

 Philosophical Transactions for 1790, being then 

 unfinished. The radius of the arc is five feet, and 

 the arc itself is of that extent to take in nine de- 

 grees on each side of the zenith. It is divided into 

 degrees, and smaller divisions of 20' each, which 

 are numbered. Each of these last is again subdi- 

 vided into four, of 5' each. The micrometer which 

 moves the telescope and arc, is graduated to se- 

 conds, and one revolution moves the arc over 

 r 10" 08'", but the scale being large, a small frac- 

 tion of a second can be easily defined. The con- 

 struction, and improvements to the zenith sector, 

 are so well known, that a minute description of it 

 here would be nnnecessary. It will therefore suffice 

 to say, that as far as so delicate an instrument can 

 be managed in a portable observatory, or travel- 

 ling tent, which never can offer the advantages of 

 a fixed, well contrived huikhng, I have every rea- 

 son to be satisfied with it. 



The time I commenced observing at Paudree sta- 

 tion was during the heavy part of the monsoon, 

 which occasioned frequent interruptions : and al- 

 though I had intended observing by at least three 

 fixed stars, I only succeeded to rny satisfaction in 

 one, which was Aldebaran. With that star I had a 

 fortunate succession for about sixteen nights ; some 

 few of those observations being less favourable than 

 the others, were rejected, and tlie rest, from which 

 the latitude was determined, appear in the following 

 tabie, arranged in the order in which they were taken. 



