580 Thj Account of a 
feet wide of the truth. We finished at low water on May 10 ; 
and therefore the height of the station above low water at 
spring tides will, no doubt, be some very few feet more. 
art. ii.. Heights of Rook’s Hill and ButserHill. With Tables 
containing the Heights of the Stations , and the mean terrestrial 
Refractions. 
At r the ground at Rook's Hill was depressed 12 14, 
Dunnose l at Butser H ill depressed - 6 10 
At Rook's r the ground at Du nnose was depressed - 7 37 
Hill l at Butser Hill elevated - 717 
At Butser f the ground at Dunnose was depressed - 12 36 
Hill i the top of a flagstaff' at R.ook’s Hill depr. 15 12 
Dunnose and Rook’s Hill 23 31'] 
Dunnose and Butser Hill 23 3 contained arcs nearly. 
Butser Hill and Rook's Hill 9 59 J 
The flagstaff at Rook’s Hill was 20 feet high. And the axis 
of the telescope about £-§- feet above the ground at each station. 
From these observations, the mean refraction between Dun- 
nose and Rook's Hill will be found 1' 58"; between Dunnose 
and Butser Hill 2' 16" ; and between Butser Hill and Rook's 
FI ill 3 g " ; which are about of the contained arcs 
respectively, as in the table. 
By the observations across the water, the ground at Rook's 
Flill would be 97 feet lower, and that at Butser Hill 131 feet 
higher than Dunnose; 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 flill, which 
is less than the former difference by 20 feet ; therefore, sup- 
posing each of the mean refractions to have produced an equal 
