Dr. Herschel's Description of a 
37 s 
of the telescope by means of the pulleys that have been de- 
scribed. On one side of the barrel is a wheel, 2 feet 3 inches 
in diameter, with 91 teeth ; and a handle with a pinion of 4 
leaves gives motion to it, when the telescope is to be lifted up 
or let down. 
The method of stringing the pulleys is expressed in fig. 37. 
The rope A, coming from the great barrel, passes successively 
over the pulleys 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 ; and from B 
goes to another barrel, T', fig. 3. which is also near the plat- 
form ttvv, the use of which will be explained hereafter. 
By the assistance of these two motions, the telescope may 
be set to any altitude, up to the very zenith ; and in order to 
have the direction of it at cojnmand, a foot quadrant of Mr. 
Bird's is fixed at the west side of the tube, near the end of it, 
inclosed in an iron case ; upon the top of which is also planted 
a finder, or night-glass, about 21 inches long, with cross wires 
in the focus. The divisions of the quadrant are indicated by a 
spirit-level, instead of a plumb-line. 
The axle, which turns the first pinion of the mechanism for 
moving the point of support, carries a pallet. This gives mo- 
tion to a small wheel with studs, contained in a machine fixed 
to the frame of the great wheel-work, and inclosed in a little 
box. The wheel with the studs carries a perpetual screw, which 
moves a central wheel, upon the axis of which is fixed an index- 
hand, that passes over a graduated plate of 140 divisions. 
Each of these divisions answers to four turns of the handle ; 
and they are large enough that a 4th part of one of them may 
be distinguished. In this manner the hand will point out how 
many turns of the handle have been made to move the telescope 
from its most backward point of support to the most forward. 
