420 SUPPLEMENT. 



the index on its axis will turn 100 times round. But, as 

 the first pinion B has only 12 leaves, and the bar A a 

 that turns it has 15 teeth in an inch, which is 12 and a 

 fourth part more ; one inch motion of the bar will cause 

 the last pinion F to turn a hundred times round, and 

 a fourth part of a hundred over and above, which is 25. 

 So that, if A a be pushed one inch, F will be turned 

 125 times round. 



A silk thread b is tied to the a?is of the pinion D, 

 and wound several times round it ; and the other end of 

 the thread is tied to a piece of slender watch-spring G 

 which is fixed into the stud H. So that, as the bar f 

 expands, and pushes the bar A a forward, the thread 

 winds round the axle, and draws out the spring ; and 

 as the bar contracts, the spring pulls back the thread, 

 and turns the work the contrary way, which pushes 

 back the short bar A a against the long bar f. This 

 spring always keeps the teeth of the wheels in contact 

 with the leaves of the pinions, and so prevents any 

 shake in the teeth. 



In the former diagram, the eight divisions of the inner 

 circle are so many thousandth parts of an inch in the 

 expansion or contraction of the bars ; which is just one 

 thousandth part of an inch for each division moved 

 over by the index. 



