Trigonometrical Survey . 717 



Art. xlv. Particulars respecting the Altitudes of the Stations. 



The height of the station on Trevose Head, above the surface 

 of the sea at low water, was determined in 1797, by levelling. 

 The transit instrument was used for the purpose ; and there is 

 reason to believe the result, 274— feet, is within a very few 

 inches of the truth. 



In the Philosophical Transactions for 1797, p. 471, the height 

 of the station on Maker Heights is said to be 402 feet ; this was 

 also found by levelling. The altitude of St. Agnes Beacon, deter- 

 mined from that station, is 599 feet ; (see the same volume and 

 page ; ) but, if the calculation be made from the base of alti- 

 tude at Trevose Head, the height of that station, above the level 

 of the sea, will be 621 feet, which gives a difference of 22 feet. 

 It must be recollected, however, that in the first result, the com- 

 putation was carried through two intermediate stations, which 

 gave three arcs, and as many mean refractions ; and, consider- 

 ing the extreme variableness to which refractions are liable, we 

 are assuredly not to consider 22 feet deviation from the truth 

 as a large quantity. 



Besides St. Agnes Beacon, the altitudes of Cadon Barrow, 

 Brown Willy, Hensbarrow, and Bodmin Down, have been de- 

 termined from that of Trevose Head. Of the remaining stations, 

 some are derived from Maker Heights, others from Dunnose : 

 most of them are mean results, that is, each station has gene- 

 rally been found two ways ; and, as it will serve to shew what 

 errors proceed from irregularity of refraction, and imperfection 

 of observation, I shall exhibit a few particulars in relation to 

 them. 

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