Forty-feet Refecting Telescope . 379 
I call this machine the bar-index. It is of eminent use in 
giving us immediately, by means of a table made for that pur- 
pose, the place of the point of support for any given altitude 
or zenith distance of the quadrant. , 
In order to come at every part of the heavens, the vertical 
motion of the telescope requires the addition of the horizontal 
one. This has been obtained by another very simple mecha- 
nism. We have already seen that the bottom frame rests upon 
20 concentric rollers, and is moveable upon a pivot. 
At ww x x y, fig. 3. is a machine in every respect like that 
which has been described as giving motion to the point of sup- 
port, except that instead of a bar with two lantern pinions, the 
great wheel here carries an iron barrel, 2 feet 8 inches long, 
and 5 inches in diameter. Near the ends of the great cross 
beam 1 1 , are planted two pulleys ; one at T, the other at V. 
Round the outer circular wall is a gravel- walk, 12 feet broad ; 
and on a grass-plot close to the margin of this walk are eight 
posts of oak, in large frames, firmly buried in the ground, at 
equal distances, so as only to shew their heads sufficiently to 
admit an iron ring and pulley to be hung upon them as occa- 
sion may require ; the middle beam also carries an iron loop 
at each end. 
A strong rope is now thrown round one of the spokes of the 
wheel, next to the barrel, which passes with one of its ends 
under the bottom of it, while the other remains at the top. 
As soon as the handle puts the wheel-work in motion, the bar- 
rel will draw both ends of the rope, but in contrary directions. 
One of the ends is then to be led to the pulley on the great 
beam at T, while the other is made to pass over that at Y ; 
but in a contrary direction. Upon the nearest post at rectangles 
