240 Gen. Roy’s Account of 
108.2- IQ 4j ,. T g c feet. Hence, as the diRance = 31 554.6 : 
‘ 2 . 
rad :: 1.95 : tang. i2".8the mean ref ration. 
For ftippofe t (fig. 12.) to be the true place of the ground, 
then the elevation TDO - the refra&ion = TD/, or 8' 37"- 
12". 8 '=%' 24 // .2 = the angle /DO; therefore /DO + ODL = 
ic/ 58A9 = /DL ; whence /L= 100.8 feet, to which adding 
5.5 feet, the height of the axis above the ground, we have 106.3 
feet, the height of the axis at T above the axis at D as before. 
Alfo, if d (fig. 13.) be the true place of the parapet, we (hall 
have the depreffion + the refraction, or OTD + D7V= QTV= 14' 
30 A3, and OTV-.OTL, or 14' 3o // . 3 -2 / 55", 6 
= the angle LTV. Hence, Ld— 109.5 feet is what the para- 
pet of the turret would be lower than the axis at T, from 
which taking 3.2 feet, the height of the axis above the para- 
pet, there remains, as before, 106.3 feet, for the difference of 
the heights of the two Rations. 
The axis of the telefcope in Dover Caftle being Feet, 
above low-water fpring tides . . 469. 
To this add the height of the axis at the 
turnpike above that at Dover . . *06.3 
We then have, for the height of the axis at the 
turnpike above low water, nearly . . 575*3 
And 5' 9 /7 .4 the contained arc of a great circle or arc of 
diftance being divided by i2 /7 .8, the mean refradtion at the two 
Rations, we have in this inflance about part for the quan • 
tity of terrejirial refraction* 
X 
Art. 
