39-i HISTORY OF PHYSICAL ASTRONOMY. 



those two opposite forces are equal, each compensates the other, and the 

 planet cannot go nearer to the sun nor further from him than a certain 

 determinate space, and thus appears balanced and floating about him." 



This is a very remarkable passage ; but it will be observed, at the 

 same time, that the author has no distinct conception of the manner 

 in which the change of direction of the planet's motion is regulated 

 from one instant to another ; still less do his views lead to any mode 

 of calculating the distance from the central body at which the planet 

 would be thus balanced, or the space through which it might approach 

 to the centre and recede from it. There is a great interval from Borelli's 

 guesses, even to Huyghens' theorems ; and a much greater to the be- 

 ginning of Newton's discoveries. 



[England) It is peculiarly interesting to us to trace the gradual 

 approach towards these discoveries which took place in the minds of 

 English mathematicians ; and this we can do with tolerable distinct- 

 ness. Gilbert, in his work, De Magnete, printed in 1600, has only 

 some vague notions that the magnetic virtue of the earth in some way 

 determines the direction of the earth's axis, the rate of its diurnal rota- 

 tion, and that of the revolution of the moon about it. 20 He died in 

 1G03, and, in his posthumous work, already mentioned (De Mundo 

 nostro Sublunari Philosophia nova, 1651), we have already a more 

 distinct statement of the attraction of one body by another. 21 " The 

 force which emanates from the moon reaches to the earth, and, in like 

 manner, the magnetic virtue of the earth pervades the region of the 

 moon: both correspond and conspire by the joint action of both, ac- 

 cording to a proportion and conformity of motions ; but the earth has 

 more effect, in consequence of its superior mass ; the earth attracts 

 and repels the moon, and the moon, within certain limits, the earth ; not 

 so as to make the bodies come together, as magnetic bodies do, but so 

 that they may go on in a continuous course." Though this phraseology 

 is capable of representing a good deal of the truth, it does not appear 

 to have been connected, in the author's mind, with any very definite 

 notions of mechanical action in detail. "We may probably say the 

 same of Milton's language : 



What if the sun 

 Be centre to the world ; and other stars, 

 By his attractive virtue and their own 

 Incited, dance about him various rounds ? 



Par. Lost, B. viii. 



«° Lib. vi. cap. 6, 7. 21 lb. ii. c. 19. 



