Forty-feet Refecting Telescope. 391 
tance, declination, or altitude, by setting it differently ; but 
in conformity with Flamsteed's British catalogue of stars, I 
have generally adopted polar distance. 
The construction of this piece is very simple. It contains 
only one barrel, for the weight and line, which gives motion 
t ) the work ; and two small index wheels. The line is con- 
ducted from the polar distance machine into the observatory 
at the bottom of the polar distance clock, where it rises up, 
and passes over the barrel. By making this revolve, it moves 
the hand upon the axle of it, which points out the minutes 
upon the dial-plate. The hand is made adjustable in the usual 
way of the minute hand of common clocks, by going upon a 
pipe, kept firm by springs. The line is of considerable length ; 
but the case of the clock being no larger than that of the 
time-piece, a set of neat and very thin pulleys, four and four, 
are used to draw the end, after its having crossed the barrel. 
It is ytqcqssary to mind, in setting these pulleys, that they 
should run upon very thin pivots, and clear one another per- 
fectly as otherwise their action might not be adequate to the 
purpose; this however is only to stretch the end of the line 
freely and sufficiently, that in passing over the barrel it may 
not make it turn about irregularly. There will be no occasion 
for a revolution of the line upon the barrel, as I have found a 
mere passage over it of sufficient effect in turning it ; for the 
hands must all be properly counterpoised. Each revolution 
answers to one degree of change in polar distance; the minutes 
are therefore pointed out by the hand it carries. The two 
small index-plates I have mentioned, are fastened upon pivots 
against the back of the dial-plate, between it and the frame of 
the barrel. They are placed so, that their edges meet not far 
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