Royal Society. 477 



occasu ad ortum; et quoniam ut dictum est motus circularis 

 naturaliter quemdam imprimit impetum ipsi mobili, quo me- 

 diante a centro removetur, atque expellitur, veluti in funda ac 

 rota observare licet, ergo, dum prsedictus planeta circulariter 

 rotatur, removebitur a centro Solis. . . Proindeque his aderunt 

 duo motus directi inter se contrarii, alter perpetuus, ac uni- 

 formis, quo planeta impulsus a propria magnetica virtute sibi 

 connaturali vero successive admovet solari corpori, alter verb dif- 

 formis,et continue decrescens, quo planeta a puncto(E) expellitur." 

 Again (p. 78) : " Ut dictum est, virtus motiva planeta? . . . com- 

 ponitur ex circulari impulsu, et gradu virtutis prementis uni- 

 formis, et ex gradu virtutis repellentis ; " and (p. 81): — " Ut 

 suppoiiamus prsedictum motuni .... pendere h magnetica im- 

 pellente virtute . . . , et motum circularem planetse circa Solem." 

 Hence, it is certainly evident that Borelli extended the idea of 

 gravitation also to the sun (as a one-sided action ) ; but then it 

 is equally evident also that he supposed the attraction of gravity 

 to be uniform, i. e. independent of distance (as may be seen more 

 at large in the passages omitted from the above quotation), — and 

 further, that he considered the motions of the planets to be the 

 result of three forces — one circular, the other centrifugal, and 

 the third attractive or centripetal. According to our present 

 notions, a force acting circularly is impossible; and a centripetal 

 force, although the expression is still used, is no force at all, but 

 only a tendency produced by the possible resolution of velocities ; 

 whilst the motions of the planets are supposed be the result 

 of an attractive force acting conjointly with certain impressed 

 tangential velocities. 



London, November 1864. 



LIX. Proceedings of Learned Societies. 



ROYAL SOCIETY. 



[Continued from p. 400.] 



November 1 7, 18G4. — Major-General Sabine, President, in the Chair. 



HPHE following communication was read : — 



JL " Comparison of Mr. De la Rue's and Padre Sccchi's Eclipse 



Photographs." By Warren De la Rue, F.R.S. 



I have stated, in the Bakerian Lecture read at the Royal Society 

 on April 10, 1862, that the boomerang (prominence E)* was not 

 depicted on Sefior Aguilar's photographs. This is true of the prints 

 which came into my hands in England. A visit to Rome in Novem- 

 ber 1862, however, afforded an opportunity for the examination of 

 the first prints which had been taken in Spain on the day of the 



* See Index Map, Plate XV. Phil. Trans. Part I. 1862. 



