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THE POPULAR SCIENCE MONTHLY. 



not always take exactly a minute to go round. This scape-wheel has 

 forty-two teeth, which is more than the usual number. If there were 

 sixty teeth, and the pendulum marked one second at each swinging, 

 the scape-wheel would turn once every minute. But this is not neces- 

 sary ; besides, the scape-wheel must be small enough for the pallets 

 to take in about nine teeth between them, and yet be able to swing 

 clear of them altogether. 



The series of wheels in Fig. 3 is called the " train." You can not 

 see the train in the clock so plainly as it is drawn in the picture, be- 



Fig. 3. 



cause one wheel is placed behind the other in order to take as little 

 room as possible. Sometimes, instead of only one wheel, B, between 

 A and C, there will be two or three wheels all of them smaller. The 

 train of wheels is then harder to move, and the weight must be heavier. 

 If the weight drops two inches in twenty-four hours, it will need a space 



