ON THE ORIGIN OF THE PLANETARY SYSTEM. 163 



upon each other, and on the assumption that no forces 

 of other kinds have any influence on their motions. 



On our earth we cannot produce such an everlast- 

 ing motion as that of the planets seems to be; for 

 resisting forces are continually being opposed to all 

 movements of terrestrial bodies. The best known of 

 these are what we call friction, resistance of the air, 

 and inelastic impact. 



Hence the fundamental law of mechanics, accord- 

 ing to which every motion of a body on which no force 

 acts goes on in a straight line for ever with unchanged 

 velocity, never holds fully. 



Even if we eliminate the influence of gravity in 

 a ball, for example, which rolls on a plane surface, we 

 see it go on for a while, and the further the smoother 

 is the path ; but at the same time we hear the rolling 

 ball make a clattering sound that is, it produces waves 

 of sound in the surrounding bodies; there is friction 

 even on the smoothest surface ; this sets the surround- 

 ing air in vibration, and imparts to it some of its own 

 motion. Thus it happens that its velocity is con- 

 tinually less and less until it finally ceases. In like 

 manner, even the most carefully constructed wheel 

 which plays upon fine points, once made to turn, goes 

 on for a quarter of an hour, or even more, but then 

 stops. For there is always some friction on the axles, 

 and in addition there is the resistance of the air, which 



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