ATTEACTION. 487 



scientific value of the question, How do bodies act on one another at a dis- 

 tance ? is to be found in the stimulus it has given to investigations into the 

 properties of the intervening medium. 



Newton, in his Principia, deduces from the observed motions of the hea- 

 venly bodies the fact that they attract one another according to a definite law. 

 This he gives as a result of strict dynamical reasoning, and by it he shews 

 how not only the more conspicuous phenomena, but all the apparent irregu- 

 larities of the celestial motions are the calculable results of a single principle. 

 In his Principia he confines himself to the demonstration and development of 

 this great step in the science of the mutual action of bodies. He says nothing 

 there about the means by which bodies gravitate towards each other. But his 

 mind did not rest at this point. We know that he did not believe in the 

 direct action of bodies at a distance. 



" It is inconceivable that inanimate brute matter should, without the mediation of something else 

 which is not material, operate upon and affect other matter without mutual contact, as it must do 

 if gravitation in the sense of Epicurus be essential and inherent in it... That gravity should be innate, 

 inherent, and essential to matter, so that one body can act upon another at a distance, through a 

 vacuum, without the mediation of anything else, by and through which their action and force may be 

 conveyed from one to another, is to me so great an absurdity, that I believe no man, who has in 

 philosophical matters a competent faculty of thinking, can ever fall into it." Letter to Bentley. 



And we also know that he sought for the mechanism of gravitation in the 

 properties of an aethereal medium diffused over the universe. 



" It appears, from his letters to Boyle, that this was his opinion early, and if he did not publish 

 it sooner it proceeded from hence only, that he found he was not able, from experiment and obser- 

 vation, to give a satisfactory account of this medium and the manner of its operation in producing 

 the chief phenomena of nature*." 



In his Optical Queries, indeed, he shews that if the pressure of this 

 medium is less in the neighbourhood of dense bodies than at great distances 

 from them, dense bodies will be drawn towards each other, and that if the 

 diminution of pressure is inversely as the distance from the dense body the 

 law will be that of gravitation. The next step, as he points out, is to account 

 for this inequality of pressure in the medium ; and as he was not able to do 

 this, he left the explanation of the cause of gravity as a problem to succeeding 

 ages. As regards gravitation the progress made towards the solution of the 

 problem since the time of Newton has been almost imperceptible. Faraday 



* Maclaurin's account of Sir Isaac Newton's discoveries. 



