Trigonometrical Survey, 727 



The refractions on the second and third arcs, I consider as 

 most accurate, on account of the great distance between the 

 stations ; and also as more to be depended on, from the circum- 

 stance of the ray generally passing 300 feet above the ground. 



The fourth arc affords another instance of the refraction 

 varying at one station, and remaining constant at the other. 

 This, no doubt, was owing to the intervention of some partial 

 stratum of air, nearer to Epwell than Broadway Beacon. The 

 refractions, deduced from these contemporary observations are 

 certainly inconclusive. The mean refractions, (neglecting the 

 fourth arc) brought under one point of view, will be as follows. 



__ '■ 



Mean height 

 of ray above 



Refraction. 



Barom. 







the sea. 



Propl. pt. 



Therm. 



1. White Horse Hill and Whiteham 



2. Arbury H. and Brill, 5 first refracs. 



3. Arbury Hill and Wendover - 



Feet. 



734 

 774 

 854 



1 



10,9 

 1 



10,6 

 1 



11,2 



in. pts. 



2 9,5 

 29,2 

 28,8 



573 

 67,8 

 58,1 



If the air had been in a quiescent state, previous to and also 

 at the times when these observations were made, it might be 

 expected that the differences of altitudes in the stations would 

 be obtained, tolerably near the truth, barometrically. The re- 

 marks in the tables appertaining to the first and second arcs, 

 shew that such opportunities offered ; but those which belong to 

 the third, prove the wind to have been fresh ; and, as the space 

 between the stations which constitute the extremities of that 

 arc is 34 miles, nearly, it is not to be expected that a true result 

 should be obtained. The differences of altitudes of the stations 

 constituting the extremities of the two first arcs, obtained by 

 means of the observed angles of elevation and depression, as well 



5 A 2 



