SIMPLE MACHINES, OR MECHANICAL POWERS. 43 



converts the slowly-acting pace of horses to the swift hum 

 of the spiked cylinder. 



Any instrument used for thus changing ot^ modifying 

 motion is called a machine, whether it be simple or com- 

 plex in its structure. Thus even a crow-har, used in lifting 

 stones from the earth, by diminishing the motion given by 

 the hand and increasing its power, may be strictly termed 

 a machine; while a harrow, which neither alters the 

 course nor changes the velocity of the foi-ce applied, may 

 with more propriety be regarded as simply an implement 

 or tool. In common language, however, these distinctions 

 are not accurately observed, and a machine is usually con- 

 sidered to be any instrument consisting of different mov- 

 ing parts. 



All machines, however complex, may be resolved into 

 two simple parts, or powers. These are, 



1. The Lever; 



2. The Inclined Plane. 



The wheel and axle, and the pulley are modified ap- 

 plications of the LEVER ; and the wedge and the screw of 

 the INCLINED PLANE, as will be shown on the following 

 pages. These six are usually termed the mechanical 

 powers. As they really do nt)t possess any power in 

 themselves, but only regulate power, the term "simple 

 machines " may be regarded as most correct. 



THE LAW OF VIRTUAL VELOCITIES. 



Before proceeding to the simple machines, it may be 

 well to explain a very important truth, which should be 

 considered as lying at the foundation of all mechanical 

 philosophy, and which renders plain and simple many things 

 w^hich would otherwise seem strange or contradictory. 

 This is, that the force required to lift any given body is 

 always in proportion to the weight of that body, taken 



