THE PERSISTENCE OF FORCE. 197 



There is, however, a further and more important reason 

 for here dealing with the truth that Force under each of 

 these forms persists. We have to examine its warrant. 



§ 61. At the risk of trying the reader's patience, we 

 must reconsider the reasoning through which the indestruc- 

 tibility of Matter and the continuity of Motion are estab- 

 lished, that we may see how impossible it is to arrive by 

 parallel reasoning at the Persistence of Force. 



In all three cases the question is one of quantity: — does 

 the Matter, or Motion, or Force, ever diminish in quantity? 

 Quantitative science implies measurement; and measure- 

 ment implies a unit of measure. The units of measure 

 from which all others of any exactness are derived, are 

 units of linear extension. Our units of linear extension are 

 the lengths of masses of matter, or the spaces between 

 marks made on the masses ; and we assume these lengths, or 

 these spaces between marks, to remain unchanged while the 

 temperature is unchanged. From the standard-measure pre- 

 served at Westminster, are derived the measures for trigo- 

 nometrical surveys, for geodesy, the measurement of terres- 

 trial arcs, and the calculations of astronomical distances, 

 dimensions, etc., and therefore for Astronomy at large. 

 Were these units of length, original and derived, irregu- 

 larly variable, there could be no celestial dynamics; nor any 

 of that verification yielded by it of the constancy of the 

 celestial masses or of their energies. Hence, persistence 

 of the space-occupying species of force cannot be proved; 

 for the reason that it is tacitly assumed in every experi- 

 ment or observation by which it is proposed to prove 

 it. The like holds of the force distinguished as 



energy. The endeavour to establish this by measurement, 

 takes for granted both the persistence of the intrinsic force 



from empty space. Divest the conceived unit of matter of the objective 

 correlate to our subjective sense of effort, and the entire fabric of physical 

 conceptions disappears. 



