VOL. LXXXr.] PHILOSOPHICAL TRANSACTIONS. 605 



ton, for which I am obliged to my astronomical friend Mr. Aubert, as a gift 

 that will always be highly esteemed. Close to it, and of the same height, is a 

 polar distance-piece, which has a dial-plate of the same dimensions with the time- 

 piece; and is also divided into 6o parts on the outside; but these are to express 

 minutes of space. Every 10th is marked with large figures, but every single 

 one is also denoted with its proper figure, in a smaller character. The degrees 

 are shown in a square opening under the centre, and change backwards and for- 

 wards as the telescope rises or falls. This piece may be made to show polar 

 distance, zenith distance, 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 to the work; and 2 small index 

 wheels. The line is conducted 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 on the 

 axle of it, which points out the minutes on the dial-plate. The hand is made 

 adjustable in the usual way of the minute hand of common clocks, by going on 

 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, 4 and 4, are used to draw the end, after its having crossed the 

 barrel. It is necessary to mind, in setting these pulleys, that they should run 

 on very thin pivots, and clear each other perfectly; 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 

 on the barrel, as I have found a mere passage over it Of sufficient effect in turn- 

 ing it; for the hands must all be properly counterpoised. Each revolution 

 answers to 1° of change in polar distance; the minutes are therefore pointed out 

 by the hand it carries. The 1 small index-plates I have mentioned, are fastened 

 on 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 from the centre of the 

 square hole in the dial-plate, for showing the degrees; and a small square portion, 

 a little more of one than the other of the 1 wheels, may therefore be seen, in 

 front of the dial-plate, through the opening in it. 



These wheels carry contrate teeth on the inside, and a small dial-plate on the 

 back. The face of the dial-plate of the wheel which presents itself at the right, 

 carries the units of the degrees; 1, 1, 3, 4, 5, 6, 7, 8, Q, O; while that on 

 the left has a blank which remains till the of the first appears. On the axle 

 of the barrel, close to the frame-plate on the outside, is fixed a long counter- 



