The Account of a 
508 
which cannot be done in the large instrument, because the 
screws are carried round with it. 
2. The diameter of the horizontal circle being only half that 
of the larger one, it follows, that the space between any two 
dots on the limb, gives :louble the number of minutes that are 
contained in the same space on the greater circle : on this ac- 
count, each revolution in the micrometer screw in the microscope 
answers to 2'; and the circle on the microscopic micrometer 
being divided into 60 parts, each division becomes equal to 
2", but for the conveniency of notation, they are numbered 
at every 5th, with 10, 20, &c. to 50, the both being marked 1, 
to denote T : the number of seconds then commencing as 
before, the whole revolution becomes 2'. The revolutions are 
counted by means of notches on one side of the field in the 
microscope, in the same manner as in those of the large in- 
strument. 
3. This instrument not being intended for determining the 
direction of the meridian, a vertical semicircle for directing the 
telescope to the pole star became unnecessary ; yet some ap- 
paratus was required, whereby small elevations or depressions 
from the horizon might be ascertained with a tolerable degree 
of precision. For this purpose, a moveable index, of about four 
inches long, is made to turn on the horizontal axis of the upper 
telescope, and so constructed, that by means of a finger screw, 
it can be fixed firmly in any position. The lower end of this 
index is furnished with a steel micrometer screw, having a 
circle on its head, divided into 100 parts, for shewing the frac- 
tional parts of a revolution, while other divisions, on a cham- 
fered edge of the index which marks the fractional parts, give 
the number of revolutions made by the micrometer screw. 
