THE LAW OF VIRTUAL VELOCITIES. 61 



language, however, these distinctions are not accurate- 

 ly observed, and a machine is usually considered to he 

 any instrument consisting of different moving parts. 



AU machines, however complex, may he resolved into 

 two simple parts, or simple machines. 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 screiv 

 of the INCLINED PLANE, as will he shown on the follow- 

 ing pages. These six are usually termed the mechan- 

 ical poivers. As they really do not possess any power 

 in themselves, but only regulate power, the term " sim- 

 ple macliines" may be regarded as most correct. 



THE LAAV OF VIRTUAL VELOCITIES. 



Before proceeding to the simple machines, it may be 

 well to explaia 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 which would otherwise seem strange or contra- 

 dictory. This is, that the force required to lift any 

 given body is always in proportion to the weight of 

 that body, taken together with the height to be raised. 

 For instance, it requires twice the force to raise two 

 pounds as to raise one pound, three times the force to 

 raise three pounds, and so forth. Also, twice as great 

 a force is needed to elevate any weight two feet as one 

 foot, or three times as great for three feet, and so on. 

 Agam, combining these together, four times as great a 

 force is required to raise two pounds to a height of two 

 feet as to raise one pound only one foot ; eight times 



