VOL. LXXXV.] PHILOSOPHICAL TRANSACTIONS. 6l 1 



the rolling beams. This affords a passage of an easy ascent to the speculum 

 carriage, which must now be brought into a proper position for rolling up. 

 When this is done, the carriage is to be tied to the axle of the point of support, 

 and by turning the bar-machine handle, the speculum with its carriage will be 

 drawn up to the foundation beams, which are l6 inches above the foundation 

 wall. Ey the time that the carriage comes near the top, there will be room for 

 6 3-inch planks that are provided, to be laid one after another on the rolling 

 beams, which will form a platform of 5 feet 10 inches by 5 feet 5, for the re- 

 ception of the carriage. But these planks must not be put down till the teles- 

 cope has been first brought back, and fixed again close to the carriage, which 

 must be sustained in its place while this is doing. Then, advancing again, the 

 platform is laid down, board by board, till completed, while at the same time the 

 carriage will be drawn upon it. 



As soon as that is safely landed, a strong rope is to be hooked into a loop, 

 fixed on the beam. This going down to a pulley with a swivel hook at the bot- 

 tom, which is put through one of the 3 handles of the speculum, returns to a 

 pulley hung on the hook. From that pulley it goes forward to a leading pulley 

 on the foundation timber. This directs the ends of the rope to the barrel, 

 which serves for the great round motion of the whole telescope. When the 

 handle of that machine is moved, the speculum will be lifted up in its carriage, 

 which being eased, must now be turned about while the mirror is yet partly 

 resting on it, so as to become parallel with the back of the tube, and close to 

 it. As soon as the mirror is fairly suspended, the carriage must be unbolted, 

 and drawn sideways from under it. At the same time the platform must be gra- 

 dually removed, that the tube may be brought back by the bar- motion, when- 

 ever the mirror is high enough to pass over the back of it. Then letting down 

 gently the round motion handle, and guiding the mirror properly, it is to be 

 placed on a small hollow square, with a sloping back, which is planted under its 

 support. The height of the square frame is such as will bring the centre of 

 the mirror into the centre of the tube ; and the sloping back receives it in going 

 down, and throws it from the back of the tube, just as much as is required to 

 make the adjustment at the top act properly. 



When the mirror is in its place, 2 loops, which are prepared, are to be 

 screwed fast to it. They contain the collets that receive the adjusting screws 

 from, the back, through the strong upper bar, and as soon as these are fastened 

 the pulleys may be unhooked, and all the apparatus that has been used removed. 

 The 6 planks are then to be raised on the same rolling bars, where a passage 

 across the work is wanted, and where they may remain till zenith sweeps require 

 them to be moved. 



The method of preserving the speculum from damp is by having a flat cover 



4 1 2 



