388 Dr. Herschei/s Description of a 
the top and bottom, by six arms each ; and screwed on so as 
to be perpendicular to the horizon. The plate on the west is 
fixed, but that on the east is adjustable, in order to be kept 
perfectly vertical on every part of its surface. One of these is 
visible against the tube, in fig. An iron roller, 1 inch thick, 
and 2-i- in diameter, is set in a strong frame, in such a manner 
as to allow the claw, which holds it, to be set to any direction ; 
where it can be afterwards fastened by a large horned nut. This 
roller is mounted upon a frame, see fig. 40. that may be drawn 
up to any altitude, and lies upon the whole set of ladders on 
the east ; where it rolls up and down on six sets of brass rollers, 
abed, ef, which are constructed as in fig. 17. This machine 
consists of a bottom frame, and a bar, gh, at rectangles to it, 
which, when the frame lies upon its rollers on the ladders, 
stands also at rectangles to them, on the lowest part of the 
frame : it is braced so as to make the greatest resistance from 
east to west. The bar carries the iron roller, which may be 
shifted to two different situations ; g almost down to the lad- 
ders, and h more elevated. The latter is used in high alti- 
tudes. The iron roller, standing out, is then turned so about 
as to be in the direction of the length of the eastern rubbing 
iron ; in which situation it is fixed by the horned nut. The 
telescope is then brought forward, or backward, by the bar 
machine, till the rubbing iron comes to be opposite the roller. 
Upon one of the braces of this same frame at i, is also 
planted the nut belonging to the lateral screw motion, which 
has been described ; and its long bar goes always with this 
machine, when it is disengaged from the observing chair, and 
is laid back at k into a secure resting place. 
On the opposite set of ladders is another machine, which 
