Vlll CONTENTS. 



CHAPTER IV. 



PAGE 



38. Much of modern science consists of attempts to " explain " 

 the whole of the facts of the physical world by reducing 

 them to particular cases of the behaviour of matter in 

 motion '. 94 



39. Thus the facts which Black succeeded in rendering intelligible 

 up to a certain point by means of the special concept of 

 heat 95 



40. Eventually proved intelligible only when taken in relation 



with facts of a mechanical order 96 



41. The development of the main ideas of Mechanics must, 

 therefore, be discussed from the critical standpoint. The 

 earliest stage may be taken to be a process of " objectifi- 

 cation"of the force which alters the shape- or size of a 

 body, and the recognition that it can be measured in terms 

 of weight 96 



42. When the case of moving bodies is reached another concept 

 of force is necessary. We begin by elaborating the notions 

 of mass and momentum as descriptive apparatus which can 

 be successfully applied to the analysis of the phenomena 

 of collision 97 



43. Using these notions, all the phenomena of impact may be 

 summarised in the formula known as the Principle of the 

 Conservation of Momentum. It remains to show that this 

 formula holds good in all cases when a force is thought of 

 as acting between two moving bodies. Thus we reach the 

 " dynamical measure of force " 99 



44. Newton first showed that the resultant force on a particle 

 (measured as rate of change of momentum) could be 

 regarded as the vector- sum of forces which represent the 

 u notice " which it is taking of the behaviour of all other 

 particles in the universe 102 



$ 45. Since the velocity of a particle is not properly a state of 

 the particle, it follows, that this "notice" refers to any 

 two (and, therefore, to all) previous configurations of the 

 particles of the universe 103 



46. That this is not an accidental result depending upon the 

 peculiar character of the Newtonian descriptive apparatus 

 is seen from the fact that it reappears in a system of 

 mechanics so fundamentally different as that of Hertz ... 104 



47. Moreover, it can be shown to characterise other provinces 

 of fact besides the mechanical. It appears to be the con- 

 dition that changes of one order may be regarded as 

 "notice " of changes of another order 105 



