VOL. LXXXV.] PHILOSOPHICAL TRANSACTIONS. 60Q 



By the arrangement of the barrels, it is easy to see that the motion is suffi- 

 ciently divided ; as many turns of the handle are wanted to pass over a small 

 space of the heavens. The method of barrels and ropes is to be preferred to 

 wheel-work, on account of the smoothness as well as silence of the motion, both 

 which in observations of this kind are highly necessary. It is true that the great 

 stress which lies on the ropes of the micrometer-motions wears them out very 

 fast, and they must therefore be carefully watched, and often renewed ; but this 

 ought to be no objection where the end to be obtained is of such consequence. 



It would not only be troublesome to the workman, but often bring on mis- 

 takes, were he to count the turns of the handle, which perhaps for hours toge- 

 ther he is moving ; a zone-clock, therefore, has been contrived to release him 

 from that care. This is a machine which is placed on a table just by the work- 

 man. It strikes a bell when he is no longer to turn one way ; that is, when 

 the telescope is come to one of the limits of the zone, which if it be after going 

 down, is called the bottom bell ; and it strikes another bell when he has made 

 the same number of turns in a contrary direction. The telescope is by this mo- 

 tion restored to its former situation, and this 2d notice is called the top bell ; which 

 marks out the other limit of the zone. These bells not only give notice to the 

 workman when he is to change, but their different sound indicates the position 

 of the telescope, and prevents mistakes. An additional precaution has been 

 used, to make the bells repeat their stroke, the very next turn, if by some mis- 

 take the workman should have been inattentive to the first notice. In a long 

 continuation of uniform intervals of sound, we may become so used to them as 

 hardly to perceive them at all ; but the coming in of an additional sound will 

 immediately rouze the proper attention. Another very necessary use which I 

 have often made of a 2d or 3d bell, is to extend the zone, either towards the 

 north or south, for some time, when notice has been given of a star that was a 

 little above or below the sweep ; for in some parts of the heavens known stars 

 are scarce, and it becomes necessary to take in all those that may be come at. 



In order to set the zone-piece to the breadth of any particular sweep, as for 

 instance 2°, we make the workman begin at the striking of the top bell, and while 

 he turns the handle till the quadrant or polar distance-piece points out a change 

 of 2°, we keep the hand of the zone-clock lifted up, that the pin may be out 

 of the holes on the dial-plate ; for which purpose also the nut in the centre must 

 be unscrewed a little, to permit it to pass freely. When the telescope has de- 

 scended 2° 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 2°. 



VOL. XVII. 4 I 



