Trigonometrical Survey. 639 



A It would have been fortunate, had the difference of latitude 

 between Black Down and Butterton, and Butterton and St. 

 Agnes Beacon, been determined in the same manner, since the 

 latitudes of all these important stations would, in that case, have 

 been found with evident accuracy ; but, whoever has leisure and 

 inclination to go through these calculations, will find that, by 

 means of the directions of meridians at Butterton and St. Agnes 

 Beacon, the latitudes of those stations maybe found to within 

 half a second. By this I mean, that, allowing the latitude of Black 

 Down to be 50 41' i3",8, the latitude of Butterton, 50 24/ 46",3, 

 will not deviate more than half a second from the truth ; and the 

 same may be said with respect to the latitude of St. Agnes, that of 

 Butterton being admitted as correct. Supposing, therefore, the 

 latitude of Greenwich to be 51 28' 40", we may rely on the 

 assurance of the latitude of St. Agnes Beacon being determined 

 within i<§s" of the truth. 



With respect to the longitudes of these stations, their accu- 

 racy entirely depends on the observations made at Dunnose 

 and Beachy Head, for determining the length of a degree of a 

 great circle perpendicular to the meridian. The truth of the 

 deduction drawn from those observations rests on their accuracy; 

 and it can scarcely be deemed presumptuous to assert, that an 

 error of more than 1" cannot have existed in either of the 

 angles. On this account, therefore, I should suppose, that the 

 difference of longitude between those stations, has been found 

 so nearly as to leave no greater error than 1". The whole of 

 the operation to which I now allude, was performed with great 

 care; the directions of the meridians having been determined by 

 means of double azimuths of the Pole Star, confirmed by com- 

 puted azimuths. In returning to the consideration of this sub- 



4N 2 



