6o6 PHILOSOPHICAL TRANSACTIONS. [ANNO 1795. 



poised contrate pallet; which at every revolution sweeps over 1 of the teeth of 

 the first wheel, of whicli there are 10. Tlie shape of the pallet must be like 

 the barb of an arrow; but more obtuse, that it may take as much time in entering 

 very obliquely into the teeth as possible, to avoid a sudden shock. The move- 

 ment will even then be found to be quite quick enough, for showing almost 

 instantly the proper degree of polar distance. But to counteract the sudden 

 stroke of the long pallet, there is over each wheel a small lever, that rests with 

 its end between the 2 uppermost teeth ; and its shape is that of a very obtuse 

 angle, such as l6o degrees. The point of the angle sinking down between the 

 2 teeth, by its slope both ways, prevents their overshooting. The lever is held 

 down with a very weak spring, the point of which touches the lever, near the 

 place of its pivot. This method will even throw back the figure on the dial, if 

 it should have been overshot a little. Care must be taken to let all this work be 

 light, that no great force may be required in the long pallet to move it. 



The first wheel in turning about carries a short pallet, of a shape similar to 

 the long one. This must be placed low enough to let the long pallet pass freely, 

 and high enough to clear the spring-lever in going over it. The pallet, on the 

 appearance of the O, strikes a tooth of the 2d wheel, and brings the figure 1 

 into view, which with the other forms 10. The 2d dial-plate has a blank, and 

 the figures 1, 2, 3, 4, 5, 6, 7, 8, Q, 10, 11, 12, engraved on its face, and pre- 

 sents 13 teeth to the pallet on the first wheel, by which the blank and figures are 

 successively brought into view, along with the succession of the units on the 

 dial-plate of the first wheel. In this manner the degrees are shown from O to 

 129, which includes the whole range of north polar distance in this latitude; 

 while at the same time they are properly subdivided into minutes. A more mi- 

 nute division was not thought necessary with this instrument, and indeed ought 

 not to be aimed at. 



The cord which gives motion to the polar distance clock is rendered a just 

 representative, or true index, of the angular movement of the telescope, in the 

 following manner. On the machine which holds the right ascension roller, is 

 an arm in an oblique direction, on which is fastened a brass slider, 3 feet 1 inch 

 long. A coarse screw passes from one end of it to the other, and is confined 

 between its shoulders. There is a handle, by which the screw being turned, a 

 small sliding plate, which carries a pulley, is drawn backwards or forwards at 

 pleasure, along the whole range of the slider. On the telescope, near the bot- 

 tom of the front edge of the eastern rubbing-plate, is a small square bar with a 

 loop on it, which is adjustable, so that it may be occasionally brought a little 

 nearer to the mouth of the telescope, or removed farther from it. The end of 

 the polar distance cord is fastened to the loop on this bar, where it remains when 

 the polar distance clock is not in use. By this means the weight which stretches 



