60 MECHANICS. 



CHAPTER IV. 



SIMPLE MACHINES, OR MECHANICAL POWERS. 



SECTION I. 

 ADVANTAGES OF MACHINES. 



The moving forces which are appHed to various use- 

 ful purposes commonly require some change in veloc- 

 ity, direction, or mode of acting before they accom- 

 plish the desired end. For example, a running stream 

 of water has a motion in one direction only ; by the 

 use of machinery, we change tliis to an alternating mo- 

 tion, as in the saw of the saw-mill, or to a rotatory or 

 whirling motion, as in the stones of a grist-mill. The 

 direct or straightforward power of a yoke of oxen is 

 made, by the employment of the plow, to produce a 

 side-motion to the sod as well as to turn it through 

 half a circle. The thrashing-machine converts the 

 slowly-acting pace of horses to the swift hum of the 

 spiked cyhnder. 



Any instrument used for thus changing or modify- 

 ing motion is called a machine^ whether it be simple 

 or complex in its structure. Thus even a crowbar^ 

 used in hfting stones from the earth, by diminishing 

 the motion given by the hand and increasing its power, 

 may be strictly termed a machine ; while a harrow^ 

 which neither alters the course nor changes the veloci- 

 ty of the force applied, may with more propriety be re- 

 garded as simply an implement or tool. In common 



