122 Sir George Shuckburgh's Account 
lei to the equator : then A C will be a portion of an horary 
circle = the refraction in declination, found by Table I. and 
Table VIII. z. CAB, the angle of the horary and vertical 
circle ; CB, the refraction in right ascension, found by Table 
I. and Table VII. DA, the correction of the meridian, found 
by Table VI. ; and DB, the correction of the time, found by 
Table V. ; and, as AB will hardly ever be found to exceed 30', 
these triangles have been considered all as plane ; making 
due allowance, in the proportion of the sine to the radius, for 
the distances of the arcs DA, BC, and DB from their respec- 
tive poles, which has been noticed at the foot of each Table * 
The refraction in altitude having been taken from Professor 
Mayer's Tables, London edition, 1770, which is calculated 
for a density of the air, expressed by 2 9,6 inches of the ba- 
rometer, and 50°of Fahrenheit's thermometer; and, for any 
other heights of the barometer and thermometer, may be 
corrected in the usual way ; making an allowance for each de- 
gree of Fahrenheit's thermometer, above or below 50°, of 
XT9C5- This correction has been deduced from the result of 
a great many observations that I made some years since with 
the manometer, described in the Philosophical Transactions 
for the year 1777, Vol. LXVII. p. 564,. The equation that 
astronomers have generally been used to adopt, from Dr. 
Bradley's Observations, is for each degree of the ther- 
mometer ; but, I think, erroneously. 
* 1 have proposed multiplying by the secant, instead of dividing by the co-sine, as 
being the readier operation, and which comes to the same thing. 
