lo Wilde, Evolution of the Mental Faculties. 



more in evidence than in the controversy respecting the 

 measure of moving force, which has exercised the minds 

 of distinguished men of science and learning for more 

 than tw^o centuries. 



Long before the time of Descartes, the common 

 experience of mankind in the mechanical arts had shown 

 that unequal weights at each end of a lever of the first 

 order balanced each other, as in the Roman and Chinese 

 steel-yards, and that the spaces through which a large 

 and a small weight oscillate are in proportion to the 

 weight and length of the lever on each side of the fulcrum 

 and have a ratio of equality. The motions of other 

 connected bodies and machines for raising weights 

 demonstrated to these ancient observers, as clearly as the 

 sun appeared to them to go round the earth, that the 

 force of a body in motion was simply as the velocity. 

 Hence the origin of the proposition enunciated by 

 Descartes in his Principia* " That when a part of matter 

 is moved with double the quickness of another, and that 

 other is twice the size of the former, that there is just 

 precisely as much motion, but no more, in the less body 

 as in the greater," Whence also we see the a priori 

 application by Descartes of the vulgar measure of the 

 moving force of connected bodies, and the principle of 

 virtual velocities, to bodies moving by the free action of 

 gravity of which he had no experience. 



Forty years later, Newton adopted in his Principia 

 Descartes' definition of the quantity of motion in a 

 moving body in the following terms : — " The quantity of 

 motion is the measure of the same arising from the 

 velocity and quantity of matter conjointly. The motion 

 of the whole is the sum of the motion of all the parts ; 

 and therefore in a body double in quantity with equal 

 * Principia PhilosophicT, part 2, §xxxvi, 1643. 



