397 
of an Arc of the Meridian. 
Among the various eye-pieces, of different magnifying powers, 
is one furnished with a prism. This, necessarily bent at right 
angles, enables the observer to see the stars without touching the 
frame. The use of it has been found convenient; but habit and 
proper caution enable the astronomer to use either of the other 
glasses. Having given this cursory and general description of 
the instrument, as seen at first view, I shall proceed to an ex- 
planation of the plates, which show its various parts. 
Particular Description of the Ze?iith Sector, 
Plate X. Represents a general section of the instrument and 
stand. AB is one of the four great uprights of the external 
mahogany frame, and CB its top, having an opening in D, for 
admission of light. The uprights consist of two strong pieces, 
firmly screwed together; each upright having seven strong 
screws, as seen in the upright AB. The top may be considered 
as a sort of square table, screwed down on the upper part of the 
frame. Between each of the two uprights is a brace, diagonally 
fixed, for strengthening the stand, as may be seen in the plate; 
and four others go horizontally, from upright to upright, for 
the purpose of still farther strengthening the whole. Across 
the bottom of the frame, and exactly in the middle of it, is a 
very strong mahogany plank, whereon rests the sector, having a 
stout straight edge bar of the same substance underneath. In the 
middle of this cross piece, as seen at E, is an apparatus of brass, 
furnished with an azimuth circle, having a hollow receptacle of 
bell-metal in the centre, in which rests, on a conical point, the 
interior mahogany frame FGHI. This brass work, which is 
strong and substantial, may be seen in Plate XIV. It is there 
represented in piano , with the bottom part of the interior stand 
MDCCCIII. 3 F 
