CHRISTIAN HUYGENS 79 



first time the dynamic importance of the centre o^ gravity, 

 and not only its original, static bearing, known since the 

 time of Archimedes. A further discovery is the importance 

 of the product of mass into velocity, to which the name of 

 quantity of motion, or momentum, was given. ^ 



A further fundamental step, especially in view of New- 

 ton's work which soon followed, was taken by Huygens in 

 determining the laws of centrifugal force, which appears in 

 rotary motion, and in fact whenever bodies of any kind move 

 in curved paths. He was not only the first to observe it, 

 but also to investigate it thoroughly. The law which he 

 found, according to which this force, which is always directed 

 away from the centre of curvature of the path, is proportional 

 to the square of the speed in the path, inversely proportional 

 to the radius of the path, and proportional to the moving 

 mass, was first published by him in his essay on the pen- 

 dulum clock (1673); but the working out only appeared, 

 together with that of the laws of collision, after his death 

 from his posthumous papers. But the deduction is based 

 on nothing more than Galileo's law of inertia, and this variety 

 of force, as a pure consequence of inertia, was so thoroughly 

 investigated by Huygens, together with its effects, that even 

 to-day there is nothing of importance to be added. 



We already find in the work of Huygens on phenomena 

 of motion the elements out of which Newton was soon able 

 to build the whole structure of mechanics, including that of 

 the heavenly bodies. 



Very diflterent in nature from these investigations of mo- 

 tion are Huygens' investigations of the nature of light. In 



^ Huygens already, in 1 669 (in the Journal des Savants) calls the ten- 

 dency to constancy of this product, summed up over a whole system of 

 bodies, 'une admirable loi de la nature.' Descartes already made use of 

 the same product in calculation, but he did not yet realise that its all- 

 round importance is bound up with a consideration not only of the 

 magnitude, but also of the direction of velocity (treating it as a vector 

 quantity, in other words), and therefore, for example, introducing veloci- 

 ties in opposite directions with opposite signs into the calculation. 



