64,0 The Account of a 



ject, I do not perceive any source of error likely to affect the 

 conclusions, unless it be that to which all astronomical obser- 

 vations, made with instruments adjusted by plumb-lines or levels, 

 are liable. In determining differences of longitude through these 

 means, the direction in which any lateral attraction must act, 

 to produce a maximum of error, is at right angles to the meri- 

 dian. If the attraction be in the plane of it, it is obvious the 

 double azimuth, although the telescope of the theodolite does 

 not move in a vertical, will nevertheless give, almost exactly, 

 the true direction of the meridian. 



The high lands about St. Catherine's Light-House, in the Isle 

 of Wight, are about six miles from Dunnose, and nearly west 

 of it ; but it does not appear that the effect of their lateral attrac- 

 tion can have produced any sensible error; since it may be 

 shewn, that the plumb-line of the sector at Schehallien would 

 have deviated only a small part of a second from the true ver- 

 tical, had the sector itself been placed at that distance from 

 the hill. Beachy Head is situated at the eastern extremity of 

 the South Downs ; a defect of matter towards the east imme- 

 diately taking place. This circumstance renders the observa- 

 tions liable to some small errors, on account of the superior 

 lateral attraction in the opposite direction ; but, notwithstanding 

 it is very probable that an error induced by either of these 

 attractions, is so very small as to render the subject scarcely 

 worth consideration, yet, as both lie the same way, it is satisfac- 

 tory to consider that they mutually tend to correct the errors 

 which may result from either ; we may, therefore, safely con- 

 clude, that i° n' 36" is very nearly the true longitude between 

 the station on Beachy Head and that on Dunnose. Under this 

 persuasion, I consider it probable that the longitude of Black 



