12 Presidential Address 



the sun, by reason of radioactivity; or 

 a locomotive, by reason of the emission of 

 steam. In fact, variable masses are the 

 commonest, for friction may abrade any 

 moving body to a microscopic extent. 



That Mass is constant is only an ap- 

 proximation. That Mass is equal to 

 ratio of Force and Acceleration is a de- 

 finition, and can be absolutely accurate. 

 It holds perfectly even for an electron 

 with a speed near that of light ; and it is 

 by means of Newton's second law that 

 the variation of Mass with Velocity has 

 been experimentally observed and com- 

 pared with theory. 



I urge that we remain with, or go back 

 to, Newton. I see no reason against 

 retaining all Newton's laws, discarding 

 nothing, but supplementing them in the 

 light of further knowledge. 



Even the laws of Geometry have been 

 overhauled, and Euclidean Geometry is 

 seen to be but a special case of more 

 fundamental generalisations. How far 

 they apply to existing space, and how far 

 Time is a reality or an illusion, and 



