THE ASTRONOMICAL VIEW OF NATURE. 343 



tion and repulsion at a distance rather received additional 

 weight and importance when, following Newton's cosmical 

 measurements, Cavendish and Coulomb, towards the end 



the mere framing of hypotheses 

 and conjectures, for which he pro- 

 fesses to have little fancy, though 

 " the heads of some great virtuosos 

 run much upon hypotheses " ; and 

 he describes his earlier speculations 

 as "guesses which I did not rely 

 on." In fact, the elaboration of 

 the theorems contained in the 

 ' Principia ' marks the transition 

 from the metaphysical to the exact 

 or scientific treatment of natural 

 phenomena. Before Newton showed 

 the far-reaching consequences, the 

 unexpected grasp of a simple mathe- 

 matical formula in combining facts 

 apparently disconnected, no one 

 could have suspected that such 

 would be possible, and it is not to 

 be wondered at that when once 

 philosophers realised the power of 

 such formulae, an opposite move- 

 ment set in through which mathe- 

 matical processes were extolled at 

 the expense of experiment and 

 observation on the one side, and 

 of philosophical reasoning on the 

 other. Newton himself never fell 

 into this error. He knew well the 

 importance of observation, and he 

 retained to the end of his life a 

 great interest in the philosophical 

 or metaphysical problems which lay 

 beyond or behind the mathemati- 

 cal statement ; he carefully distin- 

 guished between the vis gravitatis 

 and the causa gravitatis. Two other 

 great thinkers, second only to New- 

 ton himself, took up a similar posi- 

 tion to the law of gravitation. 

 Whilst firmly believing in it, they 

 considered it to be not an ultimate 

 law of nature, a causa occulta, but 

 believed that it must be possible to 

 derive it from some mechanical 

 properties of matter. The one was 

 older than Newton. It was Huy- 



gens (1629-95) who through his 

 analysis of centrifugal forces (1673) 

 had done so much to pave the way 

 for Newton's own work. In 1690, 

 after having paid a visit to England 

 in order to become more intimately 

 acquainted with Newton's work, he 

 published at Leyden his ' Discours 

 sur la Cause de la Pesanteur,' a 

 treatise which was little noticed at 

 the time, and in which he is sup- 

 posed to have revived the vortices 

 of Descartes. Those who have care- 

 fully examined it (Fritsch, ' Theorie 

 der Newton 'schen Gravitation,' &c., 

 Konigsberg, 1874 ; and Isenkrahe, 

 'Das Rathsel von der Schwerkraft,' 

 p. 87, &c. ), find that Huygens re- 

 verted to his conception of a mate- 

 rial fluid, an ether, such as he had 

 suggested for the explanation of 

 optical phenomena, " which sur- 

 rounds the earth up to very great 

 distances, which consists of the 

 minutest particles, which fly about 

 in the most different ways in all 

 directions with tearing velocity " 

 an anticipation surely of Lesage's 

 " ultramundane corpuscles. " The 

 other great thinker who, whilst 

 firmly believing in Newton's law, 

 sought for a mechanical explanation 

 of it, was Leonhard Euler (1707-83). 

 In his ether theory, to which he 

 reverts frequently, he made an 

 attempt to explain the various 

 physical agencies, among them 

 gravitation (1743, in his ' Disser- 

 tatio de Magnete, ' which received 

 in 1744 the prize offered by the 

 Paris Academy), by the pressure of 

 the ether. He admits the difficulty 

 of the problem, but insists upon the 

 necessity of finding a mechanical 

 cause for gravitation. See Isen- 

 krahe in ' Zeitschrift fur Mathe- 

 matik und Physik,' vol. xxvi. ; but 



