CHAPTER III 

 MECHANICAL PRINCIPLES OF MACHINES 



22. Why Machines Are Used. The invention of ma- 

 chines is the result of man's desire to save labor and to econo- 

 mize in the use of his own strength by utilizing, where possible, 

 the natural forces of steam, wind, water, and electricity. 

 Man possesses only a certain amount of energy. If one man 

 works so fast as to exhaust himself by the end of the day, 

 he will not accomplish so much in the long run as the work- 

 man who utilizes a little less than half his natural strength, 

 and works at about one-third his greatest working speed. 



Strength must be carefully distributed over the day's work, 

 to obtain the best results. A machine never tires and can 

 work almost constantly at its maximum practicable speed. 

 It is for this reason that machines and labor-saving devices are 

 continually being invented. These mechanical contrivances 

 are the result of the experiences of the human race. The 

 only tools that man possessed in the beginning were his hands 

 and his teeth. As time went on he found that his hands 

 and teeth were not sufficient, and he invented a club a 

 form of hammer. At later periods axes of stone, copper, 

 bronze, and steel, and later the saw, plane, square, chisel, 

 and file were invented. All these tools resulted from neces- 

 sity, experience, observation, and the intelligent desire of the 

 human race to save itself labor and toil. 



23. Tools and Machines. Tools are simple machines. 

 When they become complicated they are called machines, 



22 



