Forty-feet Refecting Telescope. 389 
carries a large spring-bolt, on the end of which is mounted an 
iron roller, exactly like that which has been described. This 
is also adjustable to any direction. The bolt is contrived in 
such. a manner as to come out of the frame, in which it runs, 
with a pressure of 34 pounds.; and it exerts, very nearly, the 
same force during the time in which it goes through every 
part of the space it describes. The construction of the springs 
is expressed in fig. 41 : abc are two iron bars, 5 feet 6 inches 
long ; jointed at b, like a pair of compasses, and fastened on a 
pivot at a\ which remains immoveable upon the frame, while 
the other end is also fastened on a pivot, fixed to the bolt, 
which carries the roller /. The bolt is 7 feet 1 inch long, and 
3 inches square. It runs in two sets of four brass rollers 
each, at g and h , which embrace it completely, and prevent 
friction as much as possible. The joint b is sustained by a brass 
roller, which runs on the iron plate Ik. Two tapering steel 
bars, or springs, l m, n 0 , are fastened against the lower ends 
ac of the iron bars; one of them is convex, at m, the other 
concave at 0; and they exert their force against each other at 
m,Qi where the convex one rests in the concavity of the other. 
There is an adjustment of the flap which carries the bolt, see 
fig. 1. by which it may be raised up, so as to become exactly 
opposite, to the roller on the east, when that is. raised to its 
highest position. 
It will now be easily perceived, that when the eastern rub- 
bing plate, in its well adjusted vertical position, is pressed 
against the right ascension roller, by a roller exactly opposite, 
and with a force sufficient to keep it firmly poised against 
that roller, a vertical motion may be given to the telescope, in 
mdccxcv. 3E 
