Astronomical Circles and other Instruments.. 435 
to obtain the radius of the instrument to be divided, and by 
continual bisection its eighth part, which is the chord of seven 
degrees ten minutes. This chord is to be faintlv set off on a 
part of the arc, and the fixed microscope being placed over 
the middle point of the arc, or zero, the division may be car- 
^ ried to five or ten minutes, precisely in the same manner as 
has already been directed for the circle. 
Finally, whether the operation of dividing is performed by 
daylight, or by the light of a lamp, I would strongly recom- 
mend the use of shades made of thin oiled paper without wire 
marks, placed very near the limb of the instrument The 
accuracy, and, if I may be allowed the expression, the luxury 
which they afford in delicate observations, can be fully appre- 
ciated only by those, who, like myself, have been in the fre- 
quent habit of using them. 
No. I. 
For a circle of two feet 
and upwards. 
will give 72 
24 
;6o 
5 
♦2 
'F 
3 
H 
?2 
O 
Z 
8 
2 
F 
No. 2. 
No. 3. 
8 
- 4 
2 
o 
o 
o 
c 
c 
o 30 
O 1 C 
will give 180 
lilO 
go 
2 
ii 
3 
30 
3 
iS 
2 
O 
32 30 
90 
- 45 
^5 
- 5 
a 30 
ro 50 
<! and 
L o 10 
360 
3 
120 
T 
12 
5 
will give iqO o 
40 o 
-80 
(See No. 1.) 
No. 4. 
For a smalltr circle. 
will give 72 
F 
H 
2 
3_£ 
2 
ii 
3 
26 
2 
ii 
2 
24® 30' 
/ 
o 
o 
o 
24 
- 12 
6 
- 2 
1 
- o 30 
^ 15 
o 
o 
London, January, 1814. 
