Dr. C. K. Akin on the History of Force, 473 



incompatible with it only so long as it is not considered that 

 what Huyghens calls quantitas motus in the latter means, with 

 reference to any two bodies proceeding to mutual impact, their 

 relative velocity (y) multiplied into the mass (fi) of the quicker 

 body, — which is an arbitrary if not incorrect estimate of, and cer- 

 tainly different from what is ordinarily (and also by Newton) 

 called, quantity of motion or momentum. 



To the passages quoted already in the Philosophical Magazine 

 from John Bernoulli's De Vera Notione Virium Vivarum, the 

 following extracts may be usefully added : — '•* La force vive, 



produite dans un corps est equivalente a cette partie 



de la cause qui s'est consumee [italics by the transcriber] 

 en la produisant ; puisque toute cause efhciente doit etre egale 

 a son effet pleinement execute" (Discours sur le Mouvement, 

 1727, Opera, vol. iii. p. 36). And, "Tout le monde regarde 

 comme un axiome incontestable, que toute cause efhciente ne 

 saurait perir, ni tout ni en partie, qu'elle ne produise un effet 

 egal a sa perte" (ibid. p. 56). 



3. As for the principle of the conservation of force in its wider 

 sense, it was first enunciated, by implication, in Huyghens's 

 Horologium Oscillatorium, published in 1673, Prop. 4 of Part 4 

 of which is as follows : — " Si pendulum e pluribus ponderibus 

 compositum, atque e quiete dimissum, partem quamcunque 

 oscillationis integrse confecerit, atque inde porro intelligantur 

 pondera ejus singula, relicto communi vinculo, celeritates acqui- 

 sitas sursum convertere, ac quousque possunt ascendere; hoc 

 facto, centrum gravitatis ex omnibus compositae, ad eandem 

 altitudinem reversum erit, quam ante inceptam oscillationem ob- 

 tinebat" (Oper. var. vol. i. p. 126). James Bernoulli generalized 

 this proposition, asserting " quod commune centrum gravitatis 

 plurium ponderum non possit ascendere altius per gravitatis 

 eorum effectum, quam unde descendit" (ibid. p. 247). 



4. Intimately connected with the principle of the Conservation 

 of Energy strictly so called (taking the word energy, according to 

 the proposition of Young, as English for vis viva) is that of the 

 Correlation of Forces, or, as it has been called by a logician of 

 great reputation, of the Allotropy of Force. There has been of 

 late a good deal of controversy regarding the priority of inven- 

 tion or discovery of this last-named principle ; and it may con- 

 sequently be interesting, in an historical point of view, to take 

 cognizance of passages of much earlier date than any hitherto 

 relied upon as establishing such priority, and upon which I have, 

 in the majority of cases, rather accidentally lighted. The follow- 

 ing is an extract from Placidus Heinrich's Die Phosphorescenz, 

 &c, published in 1812 : — 



** Unterdessen wissen wir wenigstens so viel mit Zuverlassigkeit, 

 Phil. May. S. 4. Vol. 28. No. 191. Dec. 1864. 2 I 



