112 HISTORY OF PHYSICAL ASTRONOMY. 



traction of matter, we cannot help noticing Francis Bacon ; for his 

 notions were so far from being chargeable with the looseness and indis- 

 tinctness to which we have alluded, that he proposed an experiment 20 

 which was to decide whether the facts were so or not ; whether the 

 gravity of bodies to the earth arose from an attraction of the parts of 

 matter towards each other, or was a tendency towards the centre of 

 the earth. And this experiment is, even to this day, one of the best 

 which can be devised, in order to exhibit the universal gravitation of 

 matter : it consists in the comparison of the rate of going of a clock in 

 a deep mine, and on a high place. Huyghens, in his book De Causa 

 Gravitatis, published in 1G90, showed that the earth would have an 

 oblate form, in consequence of the action of the centrifugal force ; but 

 his reasoning does not suppose gravity to arise from the mutual attrac- 

 tion of the parts of the earth. The apparent influence of the moon 

 upon the tides had long been remarked ; but no one had made any 

 progress in truly explaining the mechanism of this influence ; and all 

 the analogies to which reference had been made, on this and similar 

 subjects, as magnetic and other attractions, were rather delusive than 

 illustrative, since they represented the attraction as something- peculiar 

 in particular bodies, depending upon the nature of each body. 



That all such forces, cosmical and terrestrial, were the same single 

 force, and that this was nothing more than the insensible attraction 



' O 



which subsists between one stone and another, was a conception equally 

 bold and grand ; and would have been an incomprehensible thought, 

 if the views which we have already explained had not prepared the 

 mind for it. But the preceding steps having disclosed, between all the 

 bodies of the universe, forces of the same kind as those which produce 

 the weight of bodies at the earth, and, therefore, such as exist in every 

 particle of terrestrial matter ; it became an obvious question, whether 

 such forces did not also belong to all particles of planetary matter, and 

 whether this was not, in fact, the whole account of the forces of the 

 solar system. But, supposing this conjecture to be thus suggested, 

 how formidable, on first appearance at least, was the undertaking of 

 verifying it ! For if this be so, every finite mass of matter exerts forces 

 which are the result of the infinitely numerous forces of its particles, 

 these forces acting in different directions. It does not appear, at first 

 sight, that the law by which the force is related to the distance, will 

 be the same for the particles as it is for the. masses ; and, in reality, it 



! JYbi>. Org. Lib. ii. Aph. 36. 



