5%5 
Trigonometrical Survey. 
We take the height of Botley Hill (890 feet) a mean of 
500,885,885, which the observations at Leith Hill, Banstead, 
and Crow borough Beacon respectively produce, by making 
use of ~ of the contained arcs for refraction : this height ex- 
ceeds that in General Roy's table by 31 feet; but we are not 
certain of its being nearer the truth : only it may be remarked, 
in the table, p. 246 (Phil. Trans. Vol. LXXX.), that be- 
tween the several stations from High Nook to Botley Hill, the 
mean refractions are very great. 
From the reciprocal observations at Leith Hill, Banstead, 
and Shooter's Hill, the height of the last station is 446 feet, 
which is the same, in fact, as that obtained in the following 
manner,. General Roy found by levelling, that the floor of 
the upper story of the Bull Ifin at Shooter's Hill was 444 feet 
above the Gun Wharf at Woolwich ; and he allowed 22 feet 
for the fall to low water at the sea ; the sum is 466 feet. In 
1794, we levelled from the Inn to the Station, and found the 
latter 21 feet lower than the floor, which taken from 4 66, 
there remains 445 feet for the station's height. 
Notwithstanding this consistency, and also that in the height 
of St. Ann's Hill, found by different methods, it is evident from 
the observations at Dunnose, Rook's Hill, and Butser Hill, 
that relative heights deduced from elevations, or depressions, 
cannot always be depended upon to less than about 10 feet, 
even supposing those heights are the means of two or three 
independent results, except, perhaps, reciprocal observations 
were made exactly at the same time. The very great dif- 
ference in the observed elevations of St. Ann's Hill, proves that 
no dependance can be placed on single observations. But 
that was not the only instance ; for, at the station on Rook's 
