CHAPTER VII Continued. 



MACHINES. 



A MACHINE is any contrivance which has for its object the 

 transferring or transforming of energy. In an electric-light plant 

 the energy of the steam is communicated to the moving parts of 

 the engine, to be transferred by belts to the armatures of the 

 dynamo, where it is transformed into electric energy. In me- 

 chanics it is customary to restrict the term " machine" to such 

 devices as merely transfer energy. This is clearly the case with 

 those known as simple machines. They are the lever, the wheel 

 and axle, the pully, inclined plane, wedge and screw; all other 

 machines, however complicated, are but combinations of two or 

 more of these simple machines. 



No force is gained by the use of machines; indeed, much of the 

 force applied to the machine is wasted in overcoming friction 

 and other resistances, so that force is lost by the use of ma- 

 chines. But machines enable us to apply force advantageously; 

 enables us to employ the force of wind, water, steam or the 

 strength of animals. By the use of machines a small force mov- 

 ing rapidly may move a great body slowly, or a great force 

 moving slowly may put a small body in rapid motion. The force 

 applied to do work is called power, P. ; the resistance to be over- 

 come is called weight, W. Work consists of overcoming resist- 

 ance, 'and equals the product of the weight by the vertical height 

 to which it has been raised. Two forces, acting in opposite direc- 

 tions upon the same body, will be in equilibrium when they do 

 equal amounts of work. 



With this principle as a test, we may study with interest the 

 simple machines mentioned, considering the parts as without 

 weight and moving without friction. 



The lever is an inflexible bar, turning freely on a point called 



(63) 



