Sir George Shuckburgh’s Account 
ii 4 
manner, that the fixed wires may become a correct diameter, 
I believe the whole instrument will have been completely ad- 
justed. 
(30.) Before I conclude this account, I must beg leave to 
trouble the reader’s patience a few minutes more, in order to 
give a general idea of the manner of making the ordinary 
observations of right ascension and declination, with this in- 
strument. 
Let the telescope be adjusted towards the meridian mark, 
in such manner that the centre wire may exactly bisect it ; 
then note if the index wire of the equatorial micrometers 
bisect the points of 360° and 180°. ‘ If it does, the instru- 
ment is prepared for observing a transit ; if not, and the dif- 
ference be considerable, it must be corrected by moving the 
polar axis, by its adjusting screw. But as this quantity will 
seldom exceed 8" or 10", it will be more convenient to note 
this quantity in the journal, and allow for it afterwards in re- 
ducing the observations * As this quantity will from time to 
* This may be done by the following rule. 
_ . „ sine zenith dist. , , 
JE x sine r x — rr— = x the correction -f or — according as the tele- 
sine polar dist. 
scope points to the east or west of the meridian. 
Where JE is — the error on the equatorial circle. 
And P — the angle that the polar axis makes with a ray from the meridian mark. 
And if there should appear any error in the horizontal position of the axis of the 
declination circle at this time, by the level not standing true, that error may be cor- 
rected by this theorem. 
_ sine alt. , 
D X : 7 — x the correction. 
sine polar dist. 
Where D is — angle of depression of one end of this axis below the horizon. 
And by means of the above theorems, a table may be calculated that will give these 
corrections always, by inspection ; such a table I have computed for my own use, but 
