450 THE WORLD MACHINE 



gravitations towards the several particles of which the body of 

 the sun is composed ; and in receding from the sun decreases 

 accurately in the duplicate proportion of the distances as far as 

 the orb of Saturn, as evidently appears from the quiescence of 

 the aphelions of the planets ; nay, and even to the remotest 

 aphelions of the comets, if those aphelions are also quiescent. 

 But hitherto I have not been able to discover the cause of those 

 properties of gravity from phenomena, and I frame no hypo- 

 theses (hypotheses non fingo) ; for whatever is not deduced from 

 the phaenomena is to be called a hypothesis ; and hypotheses, 

 whether metaphysical or physical, whether of occult qualities 

 or mechanical, have no place in experimental philosophy. In this 

 philosophy, particular propositions are inferred from the phaeno- 

 mena and afterwards rendered general by induction. Thus it 

 was that the impenetrability, the mobility and the impulsive 

 force of bodies, and the laws of motion and of gravity were 

 discovered. And to us it is enough that gravity does really 

 exist, and act according to the laws which we have explained, 

 and abundantly serves to account for all the motions of the 

 celestial bodies and of our sea." 



In a celebrated letter to Sir Robert Boyle on this subject, 

 Newton gave freer range to his imaginative faculties, and 

 endeavoured to show how the phenomena of gravitation might 

 flow from the supposition of a very tenuous " aether " something 

 of the same sort of concept as physicists have since imagined 

 to explain the phenomena of light. It is of interest as affording 

 a glimpse at the workings of a great mind, as do the letters of 

 Faraday and Darwin. Let the grubbing brain that never lifts 

 its eyes into the speculative blue above, consider this curious 

 passage : 



" I will suppose aether to consist of parts differing from one 

 another in subtilty by infinite degrees ... in such manner that 

 from the top of the air to the surface of the earth, and again 

 from the surface of the earth to the centre thereof, the aether 

 is insensibly finer and finer. Imagine now any body suspended 

 in the air or lying on the earth, and the aether being by the 

 hypothesis grosser in the pores which are in the upper parts of 

 the body than in those which are in the lower parts, and that 

 grosser aether being less apt to be lodged in those pores than 

 the finer aether below, it will endeavour to get out and give way 



