47 ° 
The Account of a 
The mean refractions were found by the following rules. 
1 . Reduce the elevations, or depressions, to the place of the 
axis of the telescope at each station, by adding, or subtracting, 
as the case may require, the angle at the place of observation* 
subtended by the vertical height between the object, whose 
elevation or depression was observed, and the axis of the tele- 
scope when at that station.* 
2. Then, if both are depressions, subtract their sum from 
the contained arc, and half the remainder is the mean refrac- 
tion. 
3. If one is a depression and the other an elevation, take 
their difference. Then, if the depression is greater than the ele- 
vation, subtract the difference from the contained arc, and half 
the remainder is the mean refraction. But if the elevation is 
greatest, add the difference to the contained arc, and half the 
sum is the mean refraction. 
art. hi. Table containing the Heights of the Stations. 
Stations. 
Heights. 
Black Down 
817 feet. 
Charton Common 
582 
Little Haldon 
818, 
Rippin Tor 
1549 
Furland 
589 
* For example. At the station on Hensbarrow, the ground at Bodmin Down was 
depressed 31' 27": the distance of those stations is 47337 feet; and the axis of the 
telescope was 5! feet above the ground : therefore, as 47337 : radius : : 5! feet : tang . 
24" the angle subtended by 5 f feet at that distance; which, taken from 31' 27", 
gives 31' 3" for the depression of the place of the axis, instead of the ground. Again, 
at Bodmin Down, the ground at Hensbarrow was elevated 23' 57", to which adding 
24", we have 24' 21" for the elevation of the place of the axis. 
