Trigonometrical Survey. 545 



which place the observations were completed on the 8th. The 

 staff for connecting the observations made on the Pole Star with 

 those made on the lights fired at Hensbarrow, was placed near 

 Peranzabulo: which spot is laid down in the plan, PL XXVII. 



After these directions of meridians were determined, we pro- 

 ceeded with the survey, and from St. Agnes Beacon repaired to 

 Trevose Head, a promontory on the northern coast of Cornwall. 

 The ascent from the sea to the station on this headland being 

 very gradual and unobstructed, we took the opportunity of find- 

 ing its altitude by means of the transit instrument. The levelling 

 was begun on the 30th of May, and finished the following day ; 

 from which operation, it was found that the height of the station 

 above low water-mark was 274,2 feet; which is, probably, within 

 six inches of the truth. This base of altitude, will afford the 

 means of computing the heights of the stations in the north 

 of Devon, and also of verifying those in the western part of 

 Cornwall. (See Phil. Trans, for 1797, p. 471.) 



In giving an account of this and similar articles, it is my 

 intention merely to set forth the order in which the different 

 parts of the survey have been performed. It would be prolix, 

 and perhaps, unnecessary, to assign the reasons for the choice 

 of each station. In the present instance, however, it may 

 not be improper to observe, that a station called Black 

 Down, near Lydford, was selected for the purpose of carrying 

 distances into the north of Devon, by means of the side formed 

 by that station and Carraton Hill, The difficulty of running 

 up the series of triangles from the west, (and it might have been 

 also added, towards the north,) is mentioned in the account of 

 1797. A tract of country exists in Cornwall, possessing the same 

 characteristic features with Dartmoor, and has thrown in our 



