PRELUDE TO THE EPOCH OF NEWTON. 393 



the particles of the earth to have separate motions, which produce 

 collisions, and thus propagate 15 an " agitation of the ether," radiating 

 in all directions; and, 16 "by the rotation of the sun on its axis, con- 

 curring with its rectilinear action on the earth, arises the motion of 

 the earth about the sun." The other motions of the solar system are, 

 as we might expect, accounted for in a similar manner ; but it appears 

 difficult to invest such an hypothesis with any mechanical consistency. 



John Bernoulli maintained to the last the Cartesian hypothesis, 

 though with several modifications of his own, and even pretended to 

 apply mathematical calculation to his principles. This, however, be- 

 longs to a later period of our history ; to the reception, not to the prel- 

 ude, of the Newtonian theory. 



(Borelli.) In Italy, Holland, and England, mathematicians appear 

 to have looked much more steadily at the problem of the celestial 

 motions, by the light which the discovery of the real laws of motion 

 threw upon it. In Borelli's Theories of the Medicean Planets, printed 

 at Florence in 1666, we have already a conception of the nature of 

 central action, in which true notions begin to appear. The attraction of 

 a body upon another which revolves about it is spoken of and likened 

 to magnetic action ; not converting the attracting force into a trans- 

 verse force, according to the erroneous views of Kepler, but taking it as 

 a tendency of the bodies to meet. " It is manifest," says he, 17 " that 

 every planet and satellite revolves round some principal globe of the 

 universe as a fountain of virtue, which so draws and holds them that 

 they cannot by any means be separated from it, but are compelled to 

 follow it wherever it goes, in constant and continuous revolutions." 

 And, further on, he describes 18 the nature of the action, as a matter of 

 conjecture indeed, but with remarkable correctness. 19 " We shall ac- 

 count, for these motions by supposing, that which can hardly be denied, 

 that the planets have a certain natural appetite for uniting themselves 

 with the globe round which they revolve, and that they really tend, 

 with all their efforts, to approach to such globe ; the planets, for in- 

 stance, to the sun, the Medicean Stars to Jupiter. It is certain, 

 also, that circular motion gives a body a tendency to recede from the 

 centre of such revolution, as we find in a wheel, or a stone whirled in 

 a sling. Let us suppose, then, the planet to endeavor to approach the 

 sun; since, in the mean time, it acquires, by the circular motion, a 

 force to recede from the same central body, it comes to pass, that when 



» Art, 5. is lb. 8. " Cap. 2. « lb. 11. » P. 47. 



