MECHANICS 



53 



always to undergo the same tension at every part of its 

 length. It is practically convenient to use a pulley 

 instead of a hook or ring; the pulley being a wheel which 

 turns freely on an axle passing through its centre, with 

 a groove at its circumference for the reception of the 

 cord. Nevertheless it is the cord and not the pulley 

 which is the efficient agent in this mechanical con- 

 trivance. By various ingenious arrangements of pulleys, 

 a very small force may be made to overcome a great 

 resistance, as for instance in lifting a heavy weight. But 

 for a description of these the reader is referred to pro- 

 fessed treatises on mechanics. 



FIG. 10. 



The inclined plane is another contrivance, by means 

 of which a weight may be lifted to a certain height by 

 the application of a force less than itself. The extent 

 of the inclined surface, AB (Fig. 10), is termed the 

 length of the inclined plane. Its height is repre- 

 sented by AC, and its base by BC. 



Suppose a heavy weight W to be supported on a 

 smooth inclined plane ABC by a force (weight) P, as 

 in the figure i.e., by a cord passing over a pulley at A. 

 Then it will be seen that if P exactly balances W, and 



