PHYSICAL SCIENCE 31 



of motion may be described. In these terms we may 

 specify the configuration of any system of moving 

 bodies at any instant and specify its changes. But 

 for a complete science of mechanics another concept 

 is needed. 



Our sense of sight may give us an idea of motion, 

 but, if an external body comes in contact with us, 

 owing to relative motion, our sense of touch gives us 

 a new perception, that of force, and our muscular 

 sense gives us a rough measure of its intensity. 



It would be possible to construct a scheme of 

 mechanics with force as a third fundamental unit, 

 but, as a matter of convenience, another procedure 

 is better. The concept of mass, which, in terms of 

 consciousness, may be taken as derived from that of 

 force, is a better basis for practical mechanics, be- 

 cause it may be imagined to keep constant for any 

 one body throughout a series of changes. 



If we support two bodies so that their weights are 

 not operative place two large stones, say, on smooth 

 ice we shall find that if, being of identical sub- 

 stance, they differ in size, to set them in equal motion 

 requires either different forces or equal forces exerted 

 for different times. The same result holds with two 

 fly-wheels of the same size, one of iron and one of 

 wood. The iron wheel needs more force to set it in 

 motion, and more force to stop it when once set 

 going. 



We express these facts by saying that the mass of 



