154 MEASUREMENT OF AN ARC ON THE 



the flag-Staff at the north extremity of the line was 

 observed iriimediately after the star ; and the morn- 

 ing being perfectly clear, no unsteadiness or uncer- 

 tainty arose from the effects of the vapour, which 

 had occasioned the difference between the angles 

 on the 24th and 26th. 



When the observation was made on the 30th, a 

 blue lis^ht was fixed at the south end of the base. 



Section III. — Commencement of the operations from 

 the base. The large theodelite. 



After the completion of the base line, there re- 

 mained nothing of importance to be done until I 

 received the large instrument, which arrived in the 

 beginning of Stptembtr. I had however made an 

 excursion down the sea coast, as far as Fomlicheny, 

 for the purpose of selecting the propcrest stations 

 for determining the length of a meridional arc. 

 This and the nieasurement of a de2,-ree at ri2,ht-an- 

 gles to the meridian I considered as the first object 

 of this work: I accordingly lost no time in pro- 

 ceeding to- accomplish these desiderata. 



The instrument above alluded to was made by 

 ^Ir. Caky, and is in most respects the same as 

 that described by General Roy in the Philoso- 

 phical Transactions for the year 1790, with the 

 improvements made afterwards in the microscopes, 

 and in ar. adjustment to the vertical axis, by which 

 the circle can be moved up cr let down by means 

 of two capstan screws at the top of the axis. These 

 are mentioned in the Philosophical Transactions 

 for 179-5, 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, 



