ISO' 
Sir George Shuckburgh's Account 
Explication and Use of the following Tables. 
The three first Tables are particularly calculated for the 
use of the large equatorial instrument, for the purpose of 
clearing observations with it from the effects of refraction 
and parallax. The four last are adapted more peculiarly to 
the small or portable equatorial, such as has been noticed in 
sect. 10, and which I thought might be acceptable to such 
persons as have the good fortune to possess one of these in- 
struments. Of these I ables in their order. 
Table I. gives the correction of the refraction in north po- 
lar distance, by entering it with the altitude at the top, and the 
angle of the horary and vertical circles on the left hand side ; 
and in the common point of meeting is found a quantity, in 
seconds and decimals, that is to be added to the apparent po- 
lar distance, to give the true ; this correction is always addi- 
tive. But if the same table be entered with the angle of the 
horary and vertical circles, in the right hand column, it will 
give the refraction in right ascension, by multiplying the 
quantity here found by the secant of the declination, to be 
found in Table IV. 
Table II. gives the effect of the sun’s parallax in right as- 
cension and north polar distance, and is to be entered with 
the same arguments as Table I. ; and the parallax in right 
ascension is to be multiplied by the secant of the declination, 
as before; the sun’s horizontal parallax being assumed = 8 ", 6 . 
Table III. is a similar Table, only calculated to an hori- 
zontal parallax of 10" : so that whatever be the parallax of 
the sun or planets, this correction may readily be found, al- 
most by inspection. 
