284 Prof. Challis on Newton's "Foundation of all Philosophy." 



physical and mathematical knowledge, which allows of taking 

 up on fresh grounds the inquiry as to what is fundamental and 

 universal in the conception of force. After Newton's researches 

 in physical astronomy had established the fact that masses attract 

 each other with forces varying inversely as the square of the 

 distance separating them, 'the attraction of gravity was assumed 

 to be an inherent quality of bodies, resident in their minutest 

 parts, and acting through space without the intervention of a 

 medium. And certainly, on a first view, such an inference ap- 

 pears to be drawn from observation and experience relative to 

 masses, just as from the sensible qualities of extension and form 

 in masses it is inferred that these same qualities belong to 

 atoms. But that action at a distance is not a universal condi- 

 tion of force is proved by the modern discovery that light and 

 heat, which are modes of force, are transmitted through space 

 by the intervention of a medium. If one kind of force requires 

 a medium of transmission, why not another ? Again, it is found 

 by experience that the same portion of matter may attract or 

 repel, according to circumstances. But inherent force cannot 

 possibly be so changed by circumstances ; in the same matter it 

 must continue to be always the same. And in addition to these 

 reasons for not admitting the idea of inherent force acting at a 

 distance, the principle of deriving fundamental conceptions from 

 the indications of the senses furnishes positive argument for a 

 different idea. According to that principle, we must, in seeking 

 for the fundamental idea of force, have regard to the indications 

 of the sense of touch. Now by this sense we obtain a percep- 

 tion of force as pressure, distinct and unique, and not involving 

 the variable element of distance which enters into the conception 

 of force as derived by the sense of sight alone. Hence, according 

 to such principles as those laid down by Newton in Regula III., 

 we cannot do otherwise than conclude that the fundamental idea 

 of force is pressure. 



But where there is pressure there must be a substance which 

 presses. Newton, as we have seen, admitted the existence of a 

 subtle and all-pervading medium, to the action of which he 

 attributed various dynamical effects. And in recent times even 

 experimentalists speak familiarly of the (ether, designating by 

 that name a universal elastic fluid. The conception of an aether 

 is of very ancient date, and the evidence for its reality has been 

 gradually accumulating up to the present time ; but perhaps its 

 existence is most convincingly indicated by the explanations it 

 supplies of phenomena of light, and of their analogy in several 

 respects to phenomena of sound. Apart, however, from these 

 reasons, drawn from experience and the antecedents of science, the 

 supposition of a universal sether is demanded by the foregoing 



