656 PHILOSOPHICAL TRANSACTIONS. [aNNO 1795. 



determined, was 792 feet; which, we have no reason to suppose, is more than 

 2 or 3 feet wide of the truth. We finished at low water on May 10; and there- 

 fore the height of the station above low water at spring tides will be some very 

 few feet more. 



At f the ground at Rook's Hill was depressed 12 14 



Dunnose »- at Butser Hill depressed 6 10 



At Rook's < the ground at Dunnose was depressed 7 37 



Hill 1 at Butser Hill elevated 7 17 



At Butser j the ground at Dunnose was depressed 12 36 



Hill ) the top of a flag-staff at Rook's Hill depressed 15 i<> 



/ // 

 Dunnose and Rook's Hill 2J 31 ) 



Dunnose and Butser Hill 23 3 > contained arcs nearly. 



Butser Hill and Rooks Hill 9 59 J 



The flag-Staff at Rook's Hill was 20 feet high. And the axis of the telescope 

 about 5-i- feet above the ground at each station. From these observations, the 

 mean refraction between Dunnose and Rook's Hill will be found 1' 58"; between 

 Dunnose and Butser Hill 2' 16'; and between Butser Hill and Rook's Hill 39"; 

 which are about -,'-, -^, -r'- of the contained arcs respectively, as in the table. 



By the observations across the water, the ground at Rook's Hill would be 97 

 feet lower, and that at Butser Hill 131 feet higher than Dimnose; the sum is 

 228 feet for the difference of heights of Butser Hill and Rook's Hill, obtained 

 in this manner; but from the reciprocal observations, the ground at Rook's Hill 

 is only 208 feet lower than at Butser Hill, which is less than the former dif- 

 ference by 20 feet; therefore, supposing each of the mean refractions to have 

 produced an e(]ual error in the heights, we have 79'2 — 97 + — = 702 feet, 



for the height of Rook's Hill; and 792 + 131 — y = 916 for that of But- 

 ser Hill. From these 2 determinations, the others in table 1 have been ob- 

 tained, the stations to the westward of Dunnose excepted, by taking the mean 

 of the heights as derived from different routes. Those distinguished by an 

 asterisk, were found by taking -V of the contained arc for refraction. The re- 

 fractions at the end of table 2, obtained from the dip of the horizon, are 

 very consistent; each being nearly -^ of the contained arc. The following 

 were the observations: At Leith Hill, on July 2, 1792, at 10 in the forenoon, 

 the horizon of the sea through Shoreham Gap was depressed 30' 6'. At Rook's 

 Hill about noon on Sept. 2, 179'2j the depression of the sea, in the direction of 

 Chichester spire, was 23' 30'. At Nine Barrow Down, about noon on April 

 11, 1794, in a south direction nearly, the depression was 24' 16". The axis of 

 the telescope was about 5^ feet from the ground at each of those stations. 



