THE CONTINUITY OF MOTION. 1&5 



the establishment of certain facts having an opposite impli- 

 cation, led to inquiries which have gradually proved these 

 appearances to be illusive. The discovery that the planets 

 revolve round the Sun with undiminishing speed, raised 

 the suspicion that a moving body, when not interfered with, 

 will go on for ever without change of velocity; and sug- 

 gested the question whether bodies which lose their motion, 

 do not at the same time communicate as much motion to 

 other bodies. It was a familiar fact that a stone would 

 glide further over a smooth surface, such as ice, presenting 

 no small objects to which it could part with its motion by 

 collision," than over a surface strewn with such small objects; 

 and that a projectile would travel a far greater distance 

 through a rare medium like air, than through a dense 

 medium like water. Thus the primitive notion that moving 

 bodies had an inherent tendency to lose their motion and 

 finally stop — a notion of which the Greeks did not get rid, 

 but which lasted till the time of Galileo — began to give way. 

 It was further shaken by such experiments as those of 

 Hooke, which proved that the spinning of a top continues 

 long in proportion as it is prevented from communicating 

 motion to surrounding matter. 



To explain specifically how modern physicists interpret 

 all disappearances and diminutions of visible motion, would 

 require more knowledge than I possess and more space than 

 I can spare. Here it must suffice to state, generally, that 

 the molar motion which disappears when a bell is struck by 

 its clapper, reappears in the bell's vibrations and in the 

 waves of air they produce; that when a moving mass is 

 stopped by coming against a mass that is immovable, the 

 motion which does not appear in sound reappears as 

 molecular motion; and that, similarly, when bodies rub 

 against one another, the motion lost by friction is gained in 

 the motion of molecules. But one aspect of this general 

 truth, as it is displayed to us in the motions of masses, 

 we must carefully contemplate; for otherwise the doc- 

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