724 The Account of a 



To determine the refractions on the first arc, White Horse 

 and Whiteham Hills, we have the distance between those stations 

 = 88662,2 feet, which subtends an arc of 14/ 32" nearly. 



To determine those on the second, we have the distance be- 

 tween Brill and Arbury Hill = 146530 feet, subtending an 

 arc of 24' 3",o, : those on the third, Wendover and Arbury Hill, 

 210628 feet = 34' 35"; and, for finding the refractions from 

 the two last tables, we have the distance from Broadway Bea- 

 con to Epwell = 80611,4 feet, which subtends an arc of 

 13' 11" nearly. 



The depressions and elevations were all taken to the ground, 

 excepting those which are marked with asterisks. At White 

 Horse Hill and Whiteham Hill, lamps were used at the hours of 

 9 and 10: they were also made use of at Arbury Hill and 

 Brill at 9 o'clock. In the first instances, the lamps were placed 

 (the centres of them) 1^- feet from the bottoms of the respec- 

 tive instruments ; and in the last a|- feet. 



The height of the transit telescope above the ground was always 

 5|- feet ; therefore, an allowance must be made, at each station, 

 for the angle which that space subtends at its corresponding one ; 

 this premised, the refraction will be found from one of the two 

 following rules, viz. if A be the contained arc, and D d the ob- 

 served depressions, the quantity answering to the refraction, R, 



will be expressed by — - — == — ; or, if one of the angles should 



be an elevation, e, then R = ■ +e ~ : these rules give the 

 refractions in the following table. 



