Astronomical Circles and other Instruments, 433 
A, adjusting the cross wires, and using all the various pre- 
cautions which have been before minutely detailed. Bring the 
dot under B, and place a second dot under A: turn the circle 
till this second dot arrives at B, when if A and B are accu- 
rately at the required distance from each other, it is evident 
that a division will be seen precisely in the intersection of the 
wires of A ; but if not, the error must be corrected as before 
described, by means of the micrometer. The line of reference 
is then to be brought back to the fixed microscope, in which 
position of the circle the intersection of the wires of A will 
be accurately one third of a division, or ten minutes in ad- 
vance. Turn the circle, and bring every division both of thirty 
minutes and those newly cut, successively, as they arrive in the 
microscope, to the intersection of the wires of A, tracing a new 
division with the cutting point wherever there does not appear 
to have been one previously made, and continue this till the 
whole circle is divided into parts of ten minutes each. 
In the example of the process of dividing which I have 
given, the diameter of the circle was supposed to be two feet, 
but should the instrument be of a smaller size, the microscopes 
cannot be brought so near to each other as eight degrees, and 
it then becomes necessary to choose a different series of num- 
bers for the divisions. For this purpose, I have added a small 
table containing several series which appear to be most con- 
venient, and from among which the artist may select that 
which may best suit the size of his instrument, and his own 
ideas of accuracy. 
It may not be useless briefly to recapitulate the precautions 
which are absolutely requisite to the success of this mode of 
dividing. 
