77 
of the Equatorial Instrument. 
axis, viz. the pivot N, rests upon the strong iron support 29, 30, 
31, standing ten feet above the floor, made of massive pieces 
of cast iron, 2^ inches wide, and \ inch thick, and held firmly 
together by bolts and nuts, as in the figure. 32, and 33, are two 
iron bars nearly at right angles to each other, and at \ right 
angles to the meridian, that connect this upright support 
with the walls of the building, and, going through the bricks, 
are held fast by iron collars and nuts on the outside of the 
wall ; these bars or braces resist any tendency, from the 
weight or pressure of the instrument, to push the supporter 
29, 30, and 31, out of its upright position; and, being at right 
angles to each other, serve to keep it steady with respect to 
any lateral force that may accidentally be applied. The lower 
part is continued below the floor, and firmly fixed, with mor- 
tar and lead, into the brick work of the arch which will be 
presently described. The bottom is shut up in a box or 
plinth of mahogany, 34, 35, as has been mentioned in the 
description of the frame supporting the other end of the axis, 
at 9, 10. Near the lower end of the principal axis LN, are 
inserted 10 concentric brass cones or radii, a a, hh, cc, dd, ee, 
carrying on their extremities a graduated brass circle, of 4 feet 
diameter,* at right angles to the principal axis already de- 
scribed ; this circle has two sets of divisions, one of points, and 
one of lines, each into degrees, and every 10 minutes, and the 
intermediate minutes and seconds are read off by the micro- 
scopes W and X, with a moveable wire and micrometer 
screw, such as has been described in General Roy's Account of 
his Instrument for measuring horizontal Angles. (See Philo- 
sophical Transactions, Vol. LXXX. p. 145.). The circle just 
* More correctly 49J inches. 
