CHAPTER IV 



MACHINES 



THE adage " Necessity is the Mother of Invention " 

 is especially applicable in respect to the uses to which 

 man has put machines. Work is often accomplished 

 more easily by the use of some simple machine than in 

 any other way. Sometimes it is quite impossible to 

 accomplish a certain piece of work without the aid of a 

 machine. For example, the man who desires to lift a 

 weight of 1000 pounds finds that it is beyond his power. 

 With the aid of a lever the task is easily performed. 

 And so it is with a multitude of applications of the me- 

 chanical principles. If materials are to be raised to the 

 top of a building, a rope and pulley are used. If heavy 

 logs are to be loaded on trucks, the inclined plane is 

 used. If a building is to be lifted from its foundations, 

 the screw is used. 



The Evolution of Machines. Machines have been 

 the greatest of civilizing agencies. Primitive man knew 

 nothing concerning even the simplest machines. He 

 did not realize that he had any use for such devices. 

 'The sling was probably the first mechanical device used 

 by man. With it he could throw a stone with sufficient 

 force to make it a valuable weapon for protection and 

 for securing food. Since that time all ages have had 

 their inventors. The sling was followed by the lever 

 and the sharp bone or sharp stone, used as a cutting 



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