the 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FIFTH SERIES.] 



AUGUST 1900. 



XIII. On the Motion produced in an Infinite Elastic Solid by 

 the Motion through the Space occupied by it of a body acting 

 on it only by Attraction or Repulsion. By Lord Kelvin *. 



§ 1. HHHE title of the present communication describes a 

 J_ pure problem o£ abstract mathematical dynamics, 

 without indication o£ any idea o£ a physical application. 

 For a merely mathematical journal it might be suitable, 

 because the dynamical subject is certainly interesting both in 

 itself and in its relation to waves and vibrations. My reason 

 for occupying myself with it, and for offering it to the 

 Royal Society of Edinburgh, is that it suggests a conceivable 

 explanation of the greatest difficulty hitherto presented by 

 the undulatory theory of light ; — the motion of ponderable 

 bodies through infinite space occupied by an elastic solid |. 



§ 2. In consideration of the confessed object, and for 

 brevity, I shall use the word atom to denote an ideal 

 substance occupying a given portion of solid space, and 

 acting on the ether within it and around it, according to the 

 old-fashioned eighteenth century idea of attraction and 

 repulsion. That is to say, every infinitesimal volume A of 



* Communicated by the x^uthor, having been read before the Royal 

 Society of Edinburgh, July 16th, 1900. 



t The so-called " electro-magnetic theory of light " does not cut away 

 this foundation from the old undulatory theory of light. It adds to that 

 primary theory an enormous province of transcendent interest and 

 importance ; it demands of us not merely an explanation of all the 

 phenomena of light and radiant heat by transverse vibrations of an elastic 

 si -lid called ether, but also the inclusion of electric currents, of the 

 permanent magnetism of steel and lodestone, of magnetic force, and of 

 electrostatic force, in a comprehensive ethereal dynamics. 



Phil Mag. S. 5. Vol. 50. No. 303. Aug. 1900. O 



