54 GENERAL SCIENCE 



instrument. These simple devices enabled man to live 

 better and started him on the road to civilization. His 

 thinking resulted in better places to live, in better pro- 

 tection from his enemies, and in better food. He learned 

 that useful plants would thrive better if they were cared 

 for, and thus began the simplest forms of agriculture. 

 From this time on the development has been very rapid. 

 Man has learned to harness the various forces of nature 

 and set them to work doing the countless tasks that 

 his fancy and needs have set for them. Each genera- 

 tion has had the enormous advantage of the cumulative 

 results of the thinking of all previous generations in the 

 extent of its inventions. At present our daily life is so 

 intimately related to the mechanical world through 

 its devices for producing, collecting, and distributing 

 food, for transportation, and for communication, that the 

 very existence of many people depends upon it. 



The six simple machines are : the lever, pulley, wheel 

 and axle, inclined plane, wedge, and screw. It is some- 

 times difficult to believe that the modern machines use 

 no more than these six mechanical principles in the com- 

 plicated motions they make and in doing the wonder- 

 ful things they do, but a careful analysis will show that 

 these machines are made up entirely of combinations 

 of the six simple machines. 



The Principle of Work. A machine cannot create 

 any energy nor can it do work unless work is done upon 

 it. A machine is simply an apparatus which enables 

 us to apply force advantageously. 



When a force moves a body on which it acts, we say 

 that work has been done upon that body. The amount 

 of work done is always equal to the force multiplied by 

 the distance through which it moves the body. Thus 



