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



proportionem distantiarum" (p. 168), according to Kepler, was a 

 tangential, and not a centripetal force. 



6. Again, as regards Bouillaud, he opens cap. 12 of his Astro- 

 nomia Philoldica by the following observation (p. 21) : — " .... 

 Constat quod veritati magis aptum et congruens videtur, et quod 

 valde probabilius sit planetas, et caetera corpora coelestia per 

 propriam formam moveri, quam ab anima adsistente." He then 

 goes on to animadvert upon the views of Kepler, enunciated in 

 cap. 33 of the Astron. Nova and elsewhere. Now Kepler had 

 stated (/. c. p. 178) : — " Demonstratum est cap. 32 planetarum 

 raotus intensionem et remission em sequi proportionem distanti- 

 arum simplicem. At videtur virtus ex Sole emanans intendi et 

 remitti debere in proportions duplicatavel triplicata distantiarum 

 seu linearum effluxus"; and the chapter (34) in which this is 

 stated is headed, " Qua mensura virtus ex Sole motrix, per mundi 

 amplitudinem attenuatur." With regard to this passage, Bouil- 

 laud observes (p. 23): — "Hoc non negavit Keplerus, attamen 

 [etc.] .... Sed hsec Kepleri responsio levis admodum est. 

 Nam si in superficiali quantitate considerat illam virtutem mo- 

 tricem, necessario imminuere earn debuit in ratione dupla inter- 

 vallorum : si vero in solis lineis superiori propositioni contra- 

 dict," etc. Bouillaud closes this chapter with the following- 

 observations (p. 24) : — "Dico Solem a propria sua forma circa 

 proprium axem moveri, qua ignitus et lucidus est, caeteris vero 

 planetis nullam motus speciem imprimere, quae illos vehat, ipsos 

 vero singulos a singulis formis, quibus prsediti sunt, circumduci." 

 From the passages quoted, it appears, in the first place, that 

 Bouillaud denied altogether the existence of any reaction between 

 the sun and the planets, and, in the next place, that the force 

 regarding which Bouillaud contended that its magnitude must 

 depend on the inverse ratio of the square of distance (for reasons 

 which Kepler himself had previously fully developed) was the 

 same tangential force before assumed by Kepler, and not a cen- 

 tripetal force. It may not be superfluous to state also, re- 

 garding this same force, that, so far as it appears, Kepler supposed 

 the force to be exerted only on the part of the sun ; the sun itself 

 not being liable to any reciprocal action on the part of the planets. 



7. A third name which figures prominently in the history of 

 gravitation, after Kepler and Bouillaud, is that of Borelli. Sir 

 I. Newton wrote (see Rig. Ess. App. p. 30), " Borelli did some- 

 thing;" and Sir D. Brewster adverts more fully to his specu- 

 lations. Yet it remains to be stated more precisely what was 

 the exact " something " that "Borelli did/' 



In his TkeoriccB Mediceorum Planetarum (p. 76), Borelli states : 



" Supponamus praeditum planetam a vertigine solarium radi- 



orum in orbem ferri circa solem per circulorum peripherias ab 



