42 MR. BARLOW'S ACCOUNT OF THE CONSTRUCTION OP 



On the uppei* part of the same interior frame are fixed two cast-iron rollers, 

 on which the bearing frame rests, serving- to relieve the machine of the friction 

 that would otherwise take place when the telescope is raised or lowered. The 

 two iron rods seen on each side, turning in two eyes below and adjustable at 

 top by nuts and screws, were intended to serve as braces and to preserve 

 steadiness ; they are not, however, essential, as the instrument has every requi- 

 site stability without them. 



Such is a genei-al description of the stand ; the manner of working it will be 

 understood from what follows. 



Below the fixed cross pieces, in the middle of the swinging frame, is a double 

 fixed pulley, and to the lower part, on the inside of the sliding frame, is another 

 double pulley, which rises and falls with the sliding frame. The end of the 

 rope is fixed to the cross piece, descends and passes over one of the lower pul- 

 leys, thence over one of the upper, again descends, passes over the other lower 

 pulley, then over the upper pulley, thence again to another single fixed pulley 

 on the diagonal brace ; it then passes over the lower barrel, which is turned 

 by means of the wheel, pinion, and handle, shown in the plate. The power 

 gained by the pulleys is 4 to 1, by the wheel and pinion 4 to 1, and by the 

 barrel and handle 4 to 1, making in all 64 to 1. By these means the telescope 

 may be raised even at its heaviest purchase with great facility. 



This part of the machinery, howevei-, is only intended to bring the telescope 

 to an approximate altitude ; after Avhich, the part employed for bringing a star 

 into the field, and for following it in altitude motion, is as follows. 



At the extreme end of the bearing frame is a fixed pulley: to the back of the 

 sliding frame, another ; to the front of the same fi-ame is another similar one ; 

 and a fourth at the other end of the bearing frame. The rope is first made fast 

 at the extreme end, then passes over the pulley at the back of the sliding frame, 

 thence over the pulley at the extreme end ; whence it comes directly to the 

 upper barrel ; and £ifter a few turns about this barrel, passes to the pulley on 

 the front of the sliding frame, and returning passes over the pulley in the 

 bearing frame near the upright stancheon, then over a fixed pulley on the 

 brace : to the end of the rope is attached an iron weight of fifty-six pounds, 

 which passes through a hole in the floor and hangs suspended in a well-hole 

 below, serving thus to take in the slack of the rope, thereby keeping it always 

 tight on the barrel, and also serving as a counterpoise to the swinging frame 



