IXFRA-WORLD MECHANICS AND PHYSICS 35 



panied by the same quantity of change in many 

 other objects. 



It is well known in psychology that the eye and 

 the touch are both at work to give the infant the 

 sense of space. The notion of time is acquired 

 through the eye, the ear, and the touch. 



The notion of mass is more complex. It is 

 primarily based upon the muscular sense. It in- 

 volves a notion of volume and a notion of density 

 or intensity. The observation that mere bulk does 

 not determine the relative importance of moving 

 objects, that two objects filling the same amount 

 of space, and moving with the same speed, may 

 have very different effects upon the motion of other 

 bodies, leads to the abstraction of density, and 

 indirectly to the idea of mass. But the idea of 

 mass is not at all a generally familiar one. It is 

 illy measured by weight i.e., the force exerted 

 by the earth upon the mass. This force is pro- 

 portional to the mass ; but mass or inertia is quite 

 independent of the earth and of gravitation. 



Electricity is an abstraction still less familiar to 

 untrained minds. But it is quite as fundamental 

 as matter; indeed more so, perhaps. It involves 

 the notion of a different kind of intensity, which 

 is independent, within certc'iin limits, of the quan- 

 tity of matter present. 



Heat is another physical quantity often mcasu 

 on an independent scale. But. unlik< H.-IL- r 



