WHEEL- WORK. 



101 



(3.) By teeth or cogs. 



The third of these methods has been already considered. 



190. Uses of the First Two Ways. The first 

 method is used where no great resistance is to be overcome, 

 but where evenness of motion and freedom from noise are 

 chiefly desired. It is illustrated in some sewing-machines. 

 The second method is used when the follower is to be at 

 some distance from the driver. The friction of the belt 

 upon the wheels must be greater than the resistance to be 

 overcome. It is illustrated in most sewing-machines, and 

 in the spinning-wheel. 



191. Relation of Power to Weight De- 

 termined. The follower will revolve as many times 

 as fast as the driver, as its circumference is contained 

 times in that of the driver. The problem of finding the 

 distances passed over in a given time by the power and 

 weight, and thence the relative intensities of the power 

 and the weight, thus becomes an easy one. 



EXEKCISES. TJie Wheel and Axle. 



Remark. The circumference of a circle is 3.1416 times greater 

 than its diameter. 



