INERTIA. 45 



ance of air have comparatively little effect upon 

 the motion of a round body, such as a boy's top, 

 when it turns on its axis. A heavy disk of lead, fig. 

 41, with an axis of steel which ends in a blunt polished 

 point, spins for about three 

 quarters of an hour, if allowed to 

 move in the cavity of a watch- 

 glass. In a space from which the 

 air has been removed, it even con- 

 tinues its motion during several 

 hours. 



The earth is a rotating body FlG . 41(|m ^). 

 which meets with no friction, 



and its motion, as observed by the rising and setting of 

 the heavenly bodies, continues without interruption or 

 change. Nor is the earth's motion counteracted by the 

 resistance of the air, for the earth and its atmosphere form 

 together one body which moves as a whole in space. 



Thus a body cannot change its state without some 

 external cause, and every such cause requires a definite 

 time for producing its effect, This effect can take place 

 very rapidly, but never instantaneously, or all at once. 

 Let a body of moderate size, a stone, or a piece of wood, 

 a few centimetres in diameter, be placed upon a sheet of 

 paper ; if the paper is slowly drawn along the table, the 

 body will remain on the paper and follow its motion. 

 The force which moves the body is the friction between 

 it and the paper ; this force is small but sufficient for 

 giving a slow motion to the body. If the paper be pulled 

 with a rapid jerk, the body will not move with it; its 

 motion is imperceptible while the paper slides away 

 from underneath it. To give to the body the more 



