VOL. LXXXV,] PHILOSOPHICAL TRANSACTIONS. 607 



it in all its length from the telescope to where it is suspended in the clock-case, 

 is kept always equally exerted, and no relaxation of the cord, which ought to 

 be avoided, can take place. 



When the polar distance clock is to be used, the cord is lifted into the pulley 

 of the slider, and now goes from thence to its destination as before. The right 

 ascension roller resting against the rubbing-plate, the pulley of the slider is near 

 at hand, and the cord may easily be lifted into it. The handle is now to be 

 turned till the cord, which goes from the loop at the telescope to the pulley on 

 the slider, comes to cover a certain white line or mark on the side of the tube. 

 This line when it is first made, must be placed so as to be vertical when the ra- 

 dius of motion of the loop is a little more than 1° of elevation above the horizon. 

 The theory of this arrangement is, that when a motion in polar distance takes 

 place, the tangent and the arch may be considered as equal for a few degrees, in 

 a mechanism, which aims only at minutes. And indeed as far as 1° 20', when 

 the motion is taken equally both ways of the adjusting point, the deviation from 

 truth will not even amount to quite \". 



The cord from the pulley of the polar distance machine passes straight away 

 to where it is bent over a small pulley to one just close to it, which leads it in a 

 direct line under the polar distance clock, where it rises up to the barrel. The 

 barrel is of such a diameter, as to answer as nearly as possible to the length of 

 the cord which is drawn by the motion of the telescope over 1° of polar distance; 

 but as the utmost accuracy could not have been obtained in the make of the 

 barrel, the loop at the telescope which draws the end of the cord, may be slip- 

 ped backward or forward on its bar, which will either lengthen or shorten the 

 radius of its motion, and occasion its drawing more or less of the cord. As 

 there is a good quadrant on the telescope, there remains nothing else to obtain 

 a just position of this loop than to compare the indication of the polar distance 

 piece with that of the quadrant ; and when the former is regulated to a perfect 

 agreement with the latter, we may safely rely on the truth of its report. 



The time and polar distance pieces are placed so that the assistant sits before 

 them at a table, with the speaking-pipe rising between them ; and in this manner 

 observations may be written down very conveniently. The place of new objects 

 also may directly be noted, as their right ascension and polar distance is before 

 the assistant on the table, where nothing is required but to read them off, on 

 the signal of the observer. By a catalogue iu zones the assistant may guide the 

 observer, who is with his back to the objects he views, and who ought to have 

 notice given him of such stars as have their places well settled, in order to deduce 

 from their appearance the situations of other objects that may occur in the 

 course of a sweep. In the year 17 S3, when I began this kind of observations, 

 no catalogue of stars in zones had ever been published ; I therefore gave a pat- 



