CHAPTER X 



SIMPLE MACHINES 



196. Need of Machines. Man uses a multitude of 

 devices, or tools, to enable him to do his work to better 

 advantage. Thus, he pries a stone or a log instead of 

 lifting it. If he needs to raise stones or bricks to the top 

 of a building, he employs a pulley with a rope over it; by 

 pulling downward on the rope, he pulls the weight upward. 

 He splits logs with wedge and axe, and ploughs with a slop- 

 ing knife, or ploughshare. If he has to raise barrels into a 

 wagon, he rolls them up a sloping board instead of lifting 

 them. Wherever possible, he puts a wheel or roller under 

 a heavy object, so that he can move it without lifting it. 

 Finally, in order to fasten pieces of cloth or skin together, 

 he uses a needle or an awl. Since the thickness of this tool 

 increases very gradually, a little pushing forces the fabric 

 apart, and makes a hole for the thread. The forms of 

 tools have become more ingenious and complicated with 

 man's progress in civilization, but the simple principles 

 have been known for ages. These principles are repre- 

 sented by six simple contrivances, or machines: 



(1) The Lever. (4) The Inclined Plane. 



(2) The Pulley. (5) The Wedge. 



(3) The Wheel and Axle. (6) The Screw. 



197. Law of Machines. None of these machines nor 

 any of their improved forms can originate, or create, 



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