The Account of a 
5 8 4 
Between Mein 
Dean Hill and Wingreen 
Rook's Hill and Butser Hill 
Nine Barrow Down and Wingreen - 
Leith Hill and Ditchling Beacon - 
Mean of all the above, nearly 
Leith Hill and the Horizon 
Rook's Hill and the Horizon 
Nine Barrow Down and the Horizon 
Art. iii. Remarks on the foregoing Tables. 
The height of the ground at the station on St. Ann's Hill, 
table 1, is 240 feet ; but according to General Roy (Phil. 
Trans. Vol. LXXX. p. 232) it is 321 feet : this very great 
disagreement, however, principally arises from the variableness 
in the terrestrial refraction. In 1787, at the station near 
Hampton Poor House, the ground at St. Ann's Hill was ele- 
vated 17' 39"; but at the same station in 1792, when the axis 
of the instrument was at the same height above the ground , the 
elevation was only 8' 11". General Roy took of the con- 
tained arc for the effect of refraction, and considered the height 
of St. Ann's Hill, when deduced from that of the station near 
Hampton Poor House, as more accurate than could be obtained 
by way of the station at the Hundred Acres. But, previous to 
the survey in 1787, he found by the barometer, that the station 
on St. Ann's Hill was 200 feet higher than the Thames at 
Shepperton ; and he added 33 feet for the descent to low water 
at the sea ; the sum is 233 feet, agreeing nearly with our de- 
termination. 
Refraction. 
~ of the contained arc. 
1 5 
tt 
TT 
Te 
s 
1 2 
1 
To 
To 
To 
