472 Dr. C. K. Akin on the History of Force. 



elastick. ... If it be said, that they can lose no motion, but what they 

 communicate to other bodies, the consequence is, that in vacuo they 

 must go on and penetrate one another's dimensions. . . . Seeing there- 

 fore the variety of motion, which we find in the world, is always de- 

 creasing, there is a necessity of conserving and recruiting it by active 



principles ; such as are the cause of gravity and the cause of 



fermentation." 



I trust I shall be able to enter more fully upon some future 

 occasion into a consideration of these statements of Newton, 

 which are followed by others no less remarkable. 



2. AYith regard to Prof. Bonn's extracts, I would observe that 

 both the principles, of the conservation of energy or motion, and 

 of force, owe their first enunciation in a scientific form — at least, 

 as regards particular instances- — to Huyghens. It was Huyghens 

 who, in the Journal des Savants for March 1699 (vol. ii. p. 534), 

 first stated the following law r s regarding the collision of perfectly 

 elastic bodies :— 



(1) "La quantite du mouvement qu'ont deux corps, se peut aug-. 

 menter ou diminuer par leur rencontre ; mais il y reste toujours la 

 meme quantite vers le meme cote, en soustrayant la quantite du 

 mouvement contraire." (2) ' c La somme des produits faits de la 

 grandeur de chaque corps dur, multiplie par le quarre de sa vitesse, 

 est toujours la meme devant et apres la rencontre." 

 (See also Phil. Trans, vol. iv. p. 9.27.) In Huyghens' s posthu- 

 mous dissertation, De Motu Corporum ex Percussione (Oper. rel. 

 vol. ii.), the second of the two propositions quoted is reproduceJ ; 

 but of the first proposition only one-half is given, in these 

 words (p. 84) : — 



(3) " Corporibus duobus sibi mutuo occurrentibus non semper 

 post impactum eadem motus quantitas in utroque simul sumpto 

 conservatur quae fuit ante, sed vel augeri potest vel minui." 



It is to this and the subsequent propositions that Bernoulli, 

 wishing to disprove the anti-Leibnitzian estimate of force^ refers 

 in saying [Opera, vol. iii. p. 254), 



" Observatum est a multis, praesertim ab Hugenio motus 



quantitatem, etiam in corporibus perfecte elasticis, in immensum 

 posse augeri et minui." 



And it is apparently with the very same object as that which 

 Bernoulli had in view that Huyghens, who had adopted Leibnitz's 

 measure of force, in reproducing his proposition in the treatise last 

 quoted, omits that other portion which he had appended to it in 

 the French journal. However, as is well known, the whole of 

 proposition (1) is perfectly correct* ; and proposition (3) appears 



* Cf. Cor. 3 to Newton's Third Law of Motion : — " Quantitas motus 

 quae colligitur capiendo summam motuum factornm ad eandem partem, 

 et differentiam factornm ad contrarias, non mutatur ab actione corporum 

 inter se." 



