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There is still another and very important quahty of 

 all material bodies, called inertia. This term express- 

 es their passive state— that is, that no body (not hav- 

 ing hfe), when at rest, can move itself, nor, when in 

 motion, can stop itself. A stone can not commence 

 rolling of its own accord ; a carriage can not travel on 

 the road without being drawn ; a train of cars can not 

 commence gliding upon the rails without the power of 

 the locomotive. 



On the contrary, a body, when once set in motion, 

 will continue in motion perpetually, unless stopped by 

 something else. A cannon ball rolled upon the ground 

 continues rolling till its force is gradually overcome 

 by the resistance of the rough earth. If a polished 

 metallic globe were driven swiftly on a level and pol- 

 ished metallic plane, it would continue in motion a 

 long time and travel to a great distance ; but still the 

 extremely minute rougliness of the surfaces, with the 

 resistance of the air, would continually diminish its 

 speed until finally stopped. A wheel made to spin on 

 its axis continues till the friction at the axis and the 

 impeding force of the air bring it to rest. But if the 

 air is first removed by means of an air-pump, the mo- 

 tion will continue much longer. Under a glass re- 

 ceiver, thus exhausted, a top has been made to spin 

 for hours, and a pendulum to vibrate for a day. The 

 resistance of the air may be easily perceived by first 

 striking the edge and then the broad side of a large 

 piece of pasteboard against the air of a room. It is 

 further shown by means of an interesting experiment 



