INERTIA. EXPERIMENTS AND EXAMPLES. 



11 



INERTIA. 



An important quality of all material bodies is inertia. 

 This term expresses their passive state that is, that no 

 body (not having life), when at rest, can move itself, nor, 

 when in motion, can stop itself. A stone has not power 

 to commence rolling of its own accord ; a carriage can not 

 travel on the road without being drawn ; a train of cars 

 never commences 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 some- 

 thing else. A cannon ball rolled upon the ground moves 

 on until its force is gradually overcome by the resistance 

 of the rough earth. If a polished metallic globe w^ere driven 

 swiftly on a level and polished metallic plane, it would 

 continue in motion a long time and 

 travel to a great distance ; but still 

 the extremely minute roughness of 

 the surfaces, M'ith the resistance of the 

 air, would continually diminish its 

 speed until finally stopped. A wheel 

 made to spin on its axis revolves un- 

 til the friction at the axis and the 

 impeding force of the air bring it to 

 rest. But if the air is first removed. 

 Fans revolving in a vacuum. })j means of an air-pump, the mo- 

 tion will continue much longer. Under a glass receiver, 

 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 with the air-pump. Two 

 fan-wheels, made of sheet tin, one, a, striking the air 

 with its edges, and the other, b, with its broad faces (fig. 



