Trigon ometrical Survey. 719 



therefore, supposing that fall to be gradual and constant, all the 

 way from the latter station to the surface of the sea at Bridge- 

 water Bay, we shall get 24, feet, for the height of Lugshorn 

 Corner from the surface of the sea. The altitude of this sta- 

 tion, deduced from that of Trevose Head, is 49 feet ; and, sub- 

 tracting 3 feet from it, (the height of the bank on which the 

 instrument stood above the moor,) we get 46 feet for the height 

 of the moor at Lugshorn Corner, above the level of the sea at 

 Bridgewater Bay. But this height, supposing the fall regular, is 

 proved to be 24 feet. There is, therefore, a difference of 22 

 feet, granting the whole of this to be an error on the side of the 

 survey : but, as the general surface of the moor at Bridgewater 

 Bay is several feet above the surface of the sea, we may take a 

 moiety of 24 feet, for the error of the computed height of the 

 station at Lugshorn Corner. 



Art. xlvi. Matters relating to Refraction. 



The refractions contained in this account, like those in our 

 former Papers, tend to prove, that when rays of light pass hori- 

 zontally, and considerably distant from the surface of the earth, 

 they are less bent or refracted from their rectilinear courses, 

 than theory and opinion have laid down as fact It is very cer- 

 tain, however, that objection lies against particular conclusions 

 drawn from such data as we possess ; because the angles of 

 elevation and depression of corresponding stations are observed 

 at different times, and almost always, therefore, under different 

 circumstances ; but, with the experience and continual practice 

 of thus obtaining means of computing these refractions, although 

 we may not be able to determine the refracting power of the 

 air under given circumstances, yet, as the causes which render 



4 Z 2 



