150 JAMES CLE11K MAXWELL 



and we still find it useful in the solution of certain problems, 

 in which we employ it without danger us an artificial method. 

 Tho other analogy, between light and the vibrations of an 

 elastic medium, extends much farther, but, though its import- 

 ance and fruitf ulness cannot be over-estimated, we must 

 recollect that it is founded only on a resemblance in form 

 between the laws of light and those of vibrations. By stripping 

 it of its physical dress and reducing it to a theory of * transverse 

 alternations/ we might obtain a system of truth strictly founded 

 on observation, but probably deficient both in the vividness of 

 its conceptions and the fertility of its method. I have said 

 thus much on the disputed questions of optics, as a preparation 

 for the discussion of the almost universally admitted theory of 

 attraction at a distance. 



** \Vehave all acquired the mathematical conception of these 

 attractions. We can reason about them and determine their 

 appropriate forms or formula'. These formula have a distinct 

 mathematical significance, and their results are found to be in 

 accordance with natural phenomena. There is no formula in 

 applied mathematics more consistent with Nature than the 

 formula of attractions, and no theory better established in the 

 minds of men than that of the action of bodies on one another 

 at a distance. The laws of the conduction of heat in uniform 

 media appear at first sight among the most different in their 

 phy.sical relations from those relating to attractions. The 

 quantities which enter into them are ttmj*rtHre t faw <>f It>n1i 

 cvmtucttt'ity. The word /om? is foreign to the subject. Yet 

 we find that the mathematical laws of the uniform motion of 

 heat in homogeneous media are identical in form with those of 

 attractions vary ing inversely a.> the square of the distance. We 

 have only to substitute *>MW </ krnt for etude J nltntctiun, 

 low of knit for itccrh-rntiny efft-H of <(ttni<*ti>m at any point, 

 and tcnij>rr<tlitre for j#tt< nti<tl, and the solution of a problem in 

 attractions U transformed into that of a problem in heat. 



** This analogy between the formula! of heat and attraction 

 was, I believe, first pointed out by Professor William Thomson 

 in the CitmM'fy-' JAr'/K//ff f/<v/ J,,,n-H"t t Vol. ML 



*' Now the conduction of heat is supposed to proceed by an 



