Forty-feet Refecting Telescope. 377 
contains a pinion of 12 leaves, driving a wheel 3 feet in dia- 
meter, of 88 teeth. On the axle of this latter are fixed, upon 
a long bar, two lantern pinions of twelve leaves, at a distance 
of 3 feet 9 inches from each other, and these are confined 
down to work in the two long cut bars, which pass under 
them at that distance in iron notches, to prevent their reced- 
ing sideways. The long bars are supported by narrow slips 
of timber, pp, qcp which are extended from the front to the 
back ; as otherwise the weight of these bars would bend them 
down so as to render them unfit for action. The slips are co- 
vered, with sheet iron, that they may not be injured by friction. 
The front ends of the bars are furnished with claws, which 
keep them in. their places upon the slips. 
Two supporters of oak, 29 feet 8 inches long each, 6 inches 
broad, and 4 thick, are extended from r s near the pinions, to 
r s at the back. These are made convex at the top so as to fit 
the concavity of the iron rollers A B, fig. 34. They are also 
covered with pretty thick sheet iron, to prevent their being 
worn by the motion of the weight which is to go upon them. 
The distance from the centre of one to that of the other is 5 
feet 4;§ inches. 
These things being arranged as has been described, it ap- 
pears clearly that, when the handle of the first pinion is turned, 
the system of wheels and pinions in the machine will draw the 
bars, and consequently the point of support of the tube, for- 
ward into any required situation ; and return it back to its 
former place, by turning the same handle a different way. 
At S', fig, 3. near the platform of boards, tt vv, is placed a 
barrel, 19 inches in diameter, and 17 broad, with high sides to 
confine the long rope which draws up the point of suspension 
