Dr. C. K. Akin on the History of Force. 475 



many processes by which the form of the power may be so 

 changed that an apparent conversion of one into another takes 

 place. So we can change chemical force into electric current, 

 or the current into chemical force. The beautiful experiments 

 of Seebeck and Peltier show the convertibility of heat and elec- 

 tricity ; and others by (Ersted and myself show the convertibility 

 of electricity and magnetism" (Res. in Electr., § 207, i.). 



II. On Gravitation. 



In his 'Life of Newton/ Sir D. Brewster states (vol. i. 

 p. 268) : — " Kepler could not fail to suspect that some power 

 resided in [the sun] by which the motions of the planets were 

 produced ; and he went so far as to conjecture that this power 

 diminishes as the square of the distance of the body on which 

 it is exerted; but he immediately rejects this law in favour of 

 that of the simple distances." Again, further on (p. 282) : — 

 " Bouillaud maintained that the force of attraction must vary 

 reciprocally as the square, and not, as Kepler asserted, in the 

 simple ratio of the distance." In a similar manner, Sir I. Newton 

 stated (see Rigaud's Hist. Essay, App. p. 32) : — "Bullialdus 

 wrote that all force, respecting the sun as its centre . . . must 

 be reciprocally in a duplicate ratio of the distance from the 

 centre." A reference to the original writings of Kepler and 

 Bouillaud has suggested to me the following remarks, which, 

 as they may be of some general interest, I purpose herewith 

 publishing. 



5. In the Introduction to the Astronomia Nova, referred to also 

 by Sir D. Brewster, Kepler indeed " distinctly recognizes the 

 mutual gravitation of matter"; but he extends that notion only 

 to terrestrial bodies and the moon (see Asir. Nova, p.***4). 

 As regards the action of the sun upon the planets, Kepler states 

 (/. c. p. 185) : — " Virtus ex Sole in mundum per speciem egressa 

 rapidus quidam torrens est, qui Planetas omnes adeoque totam 

 forsan auram setheriam ab occasu in ortum rapit, se ipso non 

 aptus corpora ad Solem adducere vel ab eo longius propellere ; 

 quod esset infinite sollicitudinis opus." Again, in enumerating 

 ,the six "axioms" which account for the planetary motions 

 (p. 186), he distinguishes between a "virtus, quae ex Sole," and 

 through which every planet " de loco in locum, secundum longi- 

 tudinem zodiaci transponatur," and a " virtus quae est propria 

 Planetse," and from which he assumes " accessus Planetse ad 

 Solein et ab eo recessus oriri." Hence it is evident that Kepler 

 did not consider the idea of gravitation as applicable to the 

 action of the sun on the planets; and from cap. 33, entitled 

 " Virtutem quae Planetas movet, residere in corpore Solis," it 

 appears also that the " virtus " of which the " debilitas sequitur 



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