398 Dr. Herschel's Description of a 
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 onty be troublesome to the workman, but often 
bring on mistakes, were he to count the turns of the handle, 
which perhaps for hours together he is moving ; a zone-clock, 
therefore, has been contrived to release him from that care. 
This is a machine which is placed upon a table just by the 
workman. 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 bot- 
tom bell ; and it strikes another bell when he has made the 
same number of turns in a contrary direction. The telescope 
is by this motion restored to its former situation, and this se- 
cond 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 in- 
dicates 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 mistake 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 second or third bell, is to extend the 
^one, either towards the north or south, for some time, when 
