6lO PHILOSOPHICAL TRANSACTIONS. [ANNO 1/95. 



The ropes that come from the gallery, each bracket of which is separately drawn 

 up, go through a double pulley, hung to the top cross beam, and a double pulley 

 fastened to the upper end of the gallery bracket ; after this they pass over a sin- 

 gle pulley at the top, down to 2 barrels placed under the back of the ladders, one 

 on each side. Each barrel is moved by a handle, on the axle of which is fixed 

 a pinion of 4 leaves : this works in a wheel, on one side of the barrel, of 6l teeth, 

 and 18 inches in diameter. The barrels are 25-1- inches long, and 12 inches 

 in diameter, that the rope may not be doubled often, which might hurt the uni- 

 formity of drawing up the gallerv. They are made exactly alike, and draw an 

 equal length of rope at every stroke of the handle ; but as one of the persons 

 who draw the gallery might go on quicker than the other, each of the handles 

 strikes a bell at every turn, going up as well as going down ; the different tone 

 of the bells easily shows, by sounding in regular alternate succession, when the 

 gallery is properly moved ; which therefore may be safely done in the dark. 

 The mechanism of the bell-work at each handle is in a little box, to keep it dry, 

 but sufficiently open at the side to throw out the sound. 



The metal of the great mirror is 4Q^ inches in diameter, but on the rim is 

 an offset of |- inch broad, and 1 inch deep, which reduces the concave face of it 

 to a diameter of 48 inches of polished surface. The thickness, which is equal 

 in every part of it, remains now about 3-^ inches; and its weight, when it came 

 from the cast, was 2118 lbs. of which it must have lost a small quantity in po- 

 lishing. An iron ring, 4Q^ inches in diameter within, 4 inches broad, and 1 ; 

 thick, has at the face of it on the inside a strong bead or rim added to its thick- 

 ness, which fits the offset in the speculum, but is not quite so deep as that. A 

 cross of the same substance of iron as the ring, goes over its back, and when 

 the speculum is placed into the ring, so as to rest on the offset, the cross over 

 the back confines it in the ring. By the addition of a thin cover of sheet iron 

 on the back, and another of tin on the face, the rim makes a complete case for 

 the mirror. Three strong handles are fixed against the sides of the ring, by 

 which the speculum may be lifted horizontally, or using only one of them, ver- 

 tically, as occasion may require. 



To put the speculum into the tube, there is provided a small narrow carriage, 

 going on 2 rollers. It has upright sides, between which the speculum, when 

 suspended vertically by a crane in the laboratory, is made to pass in at one end, 

 and being let down, is bolted in. The carriage is then drawn out, rolling on 

 planks, till it comes near the back of the telescope. The tube must be put 

 back as far as the bar- machine will permit it to go. Two beams coimected to- 

 gether so as to form a parallelogram of 8 feet 6 inches long, and 2 feet broad, 

 are sloped away on one end, while the other contains 2 hooks, by which it may 

 hooked into 2 holes at the end of the foundation timber, in the middle between 



