the Trigonometrical Operation. 
241 
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Art. V. Refraction on the dijlance between Dover Cafle and 
Calais. 
Let D (fig. 14.) be the place of the axis of the telefcope, 
on the north turret of the Keep in Dover Caftle, as before ; 
G the top of the great baluftrade of the fteeple of Notre Dame 
Church at Calais ; DO the horizontal line ; and CL= SD. 
The diftance of I 3 over from ^Calais by the triangles is 
1 3745 ° ^ eet > which, allowing 61169' fathoms for a degree, 
gives 22 28 .2 for the length of the contained arc nearly . 
half of which if 14". 1 is equal to the angle QDL. 
The height of U above low water at jpring tides, 
as before, is ... . 469 feet. 
The height of G is . . . 140.5 
328.5 = CL. 
* ' 
8 i3=LDG, 
~ir 14. 1 
Therefore the difference is 
Then, as 137450 : rad :: 328.5 : tang. 
To which adding the angle ODL 
We have . • • • 19 27.1 for the 
angle ODG, or what G would be depreffed below the place of 
obfervation, if there was no fuch effeft as refra&ion. But the 
depreffion by obfervation was found to be 1 7 / 5p ;/ ; wherefore 
the difference is 28 // . 1, by which, dividing the length of 
the arc of diffance 22 7 . 28 v .2, we have in this infance 1 ^.3, or 
between ~ T th and ^ w th 'part , for the quantity of terrefrial 
refraction. 
Art. VI. Refraction on the di/lance between Aliington Knoll and 
Tenterden . 
Let K (fig. 15.) be the place of the axis of the telefcope at 
Aliington Knoll; T the top of the flag-ftaff on Tenterden 
Von. LXXX. I i Steeple ; 
