OBSERVATORY. 
90“ ; adjust the bubble by raising or de- 
pressing the polar axis (first by hand till it 
be nearly right, afterwards tighten with an 
ivory key the socket which runs on the 
arch with the polar axis, and then apply the 
same ivory key to the adjusting screw at 
the end of the said arch till the bubble 
come quite right); theti tivrn the declination 
circle to the opposite 90“ ; if tlie level be 
not then right, correct half the error by the 
aforesaid adjusting screw at the end ol the 
arch, and tiie other half error by the two 
screws whicli raise or depress the end of the 
brass rod. The polar axis remaining nearly 
liorizontal as before, and the declination 
semi-circle at 0“, adjust the bubble by the 
hour-circle ; then turn the declination semi- 
circle to 90“, and adjust the bubble by 
raising or depressing the polar axis ; then 
turn the hour-circle twelve hours ; and if 
the bubble be wrong, correct half the error 
by the polar axis, and the other half error 
by the two pair of capstan screws at the 
feet of the two supports on one side of the 
axis of motion of the telescope ; and thus 
this axis will be at right angles to the polar 
axis. The next adjustment is to make the 
centre of cross hairs remain on the same ob- 
ject, while you turn the eye-tube quite 
round by the pinion of the refraction appa- 
ratns : for this adjustment, set the index on 
the slide to the first division on the dove- 
tail ; and set the division marked 18'' on the 
refraction circle to its index; then look 
through tire telescope, and with the pinion 
turn the eye- tube quite round; and if the 
centre of the hairs does not remain on tlie 
same spot during that revolution, it must be 
corrected by the four small screws, two and 
two at a time (which you will find upon un- 
screwing the nearest end of the eye-tube 
that contains the first eye-glass) ; repeat 
tliis correction till the centre of tlie hairs 
remain on the spot you are looking at 
during an entire revolution. 
In order to make the line of colhmation 
parallel to the brass rod on which the level 
bangs, set the polar axis horizontal, and the 
declination circle to 90”, adjust the level by 
the polar axis; look thiough the telescope 
on some distant horizontal object, covered 
by the centre of the cross hairs ; then invert 
tlie telescope, which is done by turning the 
hour-circle half round ; and if the centre of 
the cross hairs does not cover the same ob- 
ject as before, correct half the error by tlie 
uppermost and lowermost of the four’ small 
screws at the eye-end of the large tube of 
the telescope; this correction will give a se- 
cond object now covered by the centre of 
the hairs, which must be adopted instead of 
the first object ; then invert the telescope 
as before ; and if the second object be not 
covered by the centre of the hairs, correct 
half the error by the same two screws which 
were used before : this correction will give 
a third object, now covered by the centre 
of the hairs, which must be adopted instead 
of the second object ; repeat this operation 
till no error remains ; then set tlie hour- 
circle exactly to twelve hours (the declina- 
tion circle remaining at 90° as before) ; and 
if the centre of the cross hairs does not 
cover the last object fixed on, set it to that 
object by the two remaining small screws 
at the eye-end of the large tube, and then 
the line of collimation will be parallel to the 
brass rod. For rectifying the nonius of 
the declination and equatorial circles, lower 
the telescope as many degrees, minuses, and 
seconds, below 0" or rE on the declination 
semi-circle as are equal to the complement 
of the latitude ; then elevate tlie polar axis 
till the bubble be horizontal, and thus the 
equatorial circle will be elevated to the co- 
latitude of the place ; set this circle to six 
hours ; adjust the level by the pinion of the 
declination circle ; then turn the equatorial 
circle exactly twelve hours from the last 
position ; and if the level be not right, cor- 
rect one-half of the error by the equatorial 
circle, and tlie other half by the declination 
circle ; then torn the equatorial circle back 
again exactly twelve hours from the last 
position ; and if the level be still wrong, 
repeat the correction as before till it be 
right, when turned to either position ; that 
being done, set the nonius of the equato- 
rial circle exactly to six hours, and the no- 
nius of the declination circle exactly to 0°. 
The principal uses of this equatorial are, 
1. To find your meridian by one observation 
only ; for this purpose, elevate the equato- 
rial circle to the co-latitude of the place, 
and set the declination semi circle to the 
sun’s declination for the day and hour of 
the day required ; then move the azimuth 
and hour circles both at the same time, ei- 
ther in the same or contrary direction, till 
you bring the centre of the cross hairs in 
the telescope exactly to cover the centre of 
the sun ; when that is done, the index of 
the hour-circle will give the apparent or 
solar time at the instant of observation ; 
and thus the time is gained, though the 
sun be at a distance from the meridian; 
then turn the hour-circle till the index 
points precisely at twelve o’clock, and 
