472 LECTURE XLIX. 



own, and converted the formula into a curve, with as many flexures and 

 reflections as the labyrinth of Daedalus, he imagines that he has depicted 

 to the senses the whole procedure of nature. Such methods may often be 

 of temporary advantage, as long as we are contented to consider them as 

 approximations, or as classifications of phenomena only ; but the grand 

 scheme of the universe must surely, amidst all the stupendous diversity of 

 parts, preserve a more dignified simplicity of plan and of principles, than 

 is compatible with these complicated suppositions. 



" To show," says Newton, in the preface to the second edition of his 

 Optics, " that I do not take gravity for an essential property of bodies, I 

 have added one question concerning its cause, choosing to propose it by way 

 of a question, because I am not yet satisfied about it for want of experi- 

 ments." In the query here mentioned, he proceeds from the supposition of 

 an elastic medium, pervading all space ; a supposition which he advances 

 with considerable confidence, and which he supports by very strong argu- 

 ments, deduced as well from the phenomena of light and heat, as from the 

 analogy of the electrical and magnetic influences. This medium he supposes 

 to be much rarer within the dense bodies of the sun, the stars, the planets, 

 and the comets, than in the empty celestial spaces between them, and to 

 grow more and more dense at greater distances from them, so that all these 

 bodies are naturally forced towards each other by the excess of pressure. 



The effects of gravitation might be produced by a medium thus consti- 

 tuted, if its particles were repelled by all material substances with a force 

 decreasing, like other repulsive forces, simply as the distances increase ; its 

 density would then be every where such as to produce the appearance of an 

 attraction varying like that of gravitation. Such an ethereal medium 

 would therefore have the advantage of simplicity, in the original law of 

 its action, since the repulsive force which is known to belong to all matter, 

 would be sufficient, when thus modified, to account for the principal pheno- 

 mena of attraction. 



It may be questioned whether a medium, capable of producing the effects 

 of gravitation in this manner, would also be equally susceptible of those 

 modifications which we have supposed to be necessary for the transmission 

 of light. In either case it must be supposed to pass through the apparent 

 substance of all material bodies with the most perfect freedom, and there 

 would, therefore, be no occasion to apprehend any difficulty from a retard- 

 ation of the celestial motions ; the ultimate impenetrable particles of matter 

 being perhaps scattered as thinly through its external form as the stars 

 are scattered in a nebula, which has still the distant appearance of a 

 uniform light and of a continuous surface : and there seems no reason 

 to doubt the possibility of the propagation of an undulation through the 

 Newtonian medium with the actual velocity of light. It must be remem- 

 bered that the difference of its pressure is not to be estimated from the 

 actual bulk of the earth or any other planet alone, but from the effect of the 

 sphere of repulsion of which that planet is the centre ; and we may then de- 

 duce the force of gravitation from a medium of no very enormous elasticity. 



We shall hereafter find that a similar combination of a simple pressure 



