Forty-feet Reflecting Telescope. 401 
plate ; for which purpose also the nut in the centre must be 
unscrewed a little to permit it to pass freely. When the tele- 
scope has descended two degrees the workman must stop the 
handle. We then lift the hand to the place where the first 
pin strikes the lever of the bottom bell. Here we let the pin 
drop into its proper hole, and screw fast the central nut. 
When this is done, the workman may turn backwards and 
forwards from bell to bell, and the telescope will perform the 
required' motion of two degrees. 
The work of the zone-piece is arranged in such a manner 
as to make the numbers on the dial-plate answer to turns of 
the working handle : this however, though convenient, is not 
absolutely necessary. The number of turns to a degree varies 
a little in different altitudes ; but by trial a table may be- 
made, which will shew with sufficient accuracy the figure on 
the dial-plate to which the hand must point, that the zone- 
piece may give any required breadth to a sweep, at any certain 
polar distance^ 
By means of the speakings pipe the workman may be di- 
rected to begin, to stop, to go fast, or slow. And these, with 
a very few other orders, will be all that are wanted ; which 
being known to him and to the assistant, will occasion no mis- 
take, notwithstanding the pipes which go into the two apart- ’ 
ments are united. 
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 single pulley at the top, down to two barrels placed under 
the back of the ladders, one on each side. Each barrel is moved 
