86 GREAT MEN OF SCIENCE 



Force is every cause of change of velocity. Weight is the 

 force of gravity, which acts upon the body in question on the 

 earth. The fact that mass and weight are proportional to 

 one another for all bodies, is clearly stated as an experimental 

 fact.^ The fundamental experimental facts had already been 

 recognised by Stevin and GaUleo, in the equal rate of fall of 

 all bodies, and in a more refined form by GaHleo in his dis- 

 covery of the equal rate of oscillation of all pendulums of 

 equal length. ^ The latter fact was tested by Newton with 

 still greater accuracy than Galileo, using pendulums of 

 material of such different specific gravity and nature as gold, 

 silver, lead, glass, sand, rock salt, wood, water, wheat, and he 

 found it to be confirmed in every case.^ In these experi- 

 ments he took into account the frictional resistance experi- 

 enced by pendulums, in common with all moving bodies, at 

 least in the air, whereby he brings by way of preface a compre- 

 hensive investigation, which itself has become of fundamen- 

 tal importance, concerning frictional forces in various media. 



A second great advance of Newton's is the clear statement 

 of three laws of motion for all matter. The first two are: 

 (i) Galileo's law of inertia and (2) Galileo's law of the pro- 

 portionality between force and acceleration, whereby how- 

 ever Newton, instead of taking acceleration, that is the change 

 of velocity in unit time, takes change of momentum, that is to 

 say, the change in unit time of the product of mass and velo- 

 city, which had appeared since Huygens' investigation as a 

 measure of the effect of forces. It is clear that this refine- 

 ment, introduced by Newton as compared with Galileo, is 

 only of importance as regards variable masses (and also 

 moments of inertia). But it is also true in this case even for 

 non-material masses, as has been shown by the very latest 

 experimental results.* It is directly clear that the law of 



* Principia lib. i , def. i . ^ Dialogues, first day . ^ Principia lib. 3 , prop. 6. 



* See my statement in Vber Aether und Uraether, 2nd Ed. (Leipzig, 

 1922), p. 48. 



