ii3 
of the Equatorial Instrument. 
4 times as great as the space occupied by 10' on the limb of 
the circle ; and if the radius of the circle be 2 feet, an arc 
of io' will be equal to 0,07 inch nearly, on the limb ; and = 
0,28 inch on the scale, viz. = to the same arc on a circle of 
8 feet radius ; and if each revolution of the micrometer screw 
be intended to describe 1', the screw must contain about 35 
threads in an inch. But as it would be difficult to adjust the 
screw exactly to the scale, tile advantage of the construction 
of these micrometers is, the scale may at any time be ad- 
justed to the screw ; for let the interval between any two 
nearest divisions, — 10', on the limb, be measured by the screw, 
and suppose, instead of being = 10' or 600", it appears only 
= 570" ; it is evident, that the scale is bigger than it should 
be, or, which is the same thing, that the image is less by 
or In this case increase the distance between the 
micrometer wires, and the object glass, =~, by unscrewing 
or drawing out the tube, that carries the micrometer and eye 
glasses, and the scale is adjusted. It will at the same time, 
however, be necessary to re-adjust the object glass of the mi- 
croscope to distinct vision, by the screw of the cell, that con- 
tains it, until the image and the wires have no relative 
change of place by any motion of the eye. This will again 
occasion some small alteration in the scale, and must be cor- 
rected by repeated trials, and the scale adapted to the divi- 
sions on the arc ; and if the moveable wire of the microscope 
be now brought to coincide exactly with the fixed one, and 
the moveable index (with the mark ^.) brought to zero ( o ) 
on the screw head, the micrometer is completely adjusted. 
This having been done with all the microscopes, and the op- 
posite ones being made to agree, each with the other, in such 
MDCCXCIII, Q 
