of the Equatorial Instrument. .105 
The axis of the declination circle is then brought nearly 
horizontal, by its proper level, viz. is turned round about the 
polar axis, till the bubble of the level stands true between the 
indexes ; the instrument is then turned half round about 
the polar axis, = 180°, shewn by the microscope W. If the 
bubble then stand true, it requires no correction, but if it do 
not, correct half the error, by moving the equatorial circle by 
its handle t, and the other half by the capstan screw a; (Tab. 
XIV. fig. 1) then turn the instrument back again to its first 
position, and see if the level stand true ; if not, repeat this 
operation till it does, correcting one half of the error by the 
equatorial handle, and the other half by the screw a. The 
declination axis will then be parallel to the level, and both of 
them to the horizon. It must be remarked, that in this ope* 
ration it will be necessary to move the declination circle 
round its own axis a little, in order to bring the same side of 
the level uppermost ; but this in no degree affects the result, 
for the imaginary line, round which this axis revolves, is what 
is meant all along by the axis, and is the line to which the 
parallelism of the level is referred. 
The declination axis remaining in an horizontal position, 
with the level above the axis, as in Tab. IX. turn the de- 
clination circle 180°, viz. till the level become below the 
axis ; then, by means of the pinion b, restore the tube of 
the level to an upright position, and see if the bubble stand 
true ; if not, correct \ the error by the screw c, and the 
•other \ by a. Now turn the declination circle 90° each 
way from its last situation, and repeat the examination of the 
bubble, and correct, as before, \ by the screw h , at right 
angles to c, and \ by the screw i; and if, after all these 
P 
MDCCXCIII, 
