208 Prof. Challis on Hydrodynamics. 



those which depend only on properties of the setherial medium 

 in which the undulations are supposed to be generated and pro- 

 pagated. In three subsequent articles, contained in the Numbers 

 for December 1863, June 1864, and the Supplementary Num- 

 ber of December 1864, I have employed the same theorems in 

 investigations relating to double refraction and dispersion. The 

 theories of phenomena of this class require hypotheses to be 

 made respecting the forms and properties of the ultimate con- 

 stituents of visible and tangible substances, and are in that 

 respect distinguished from those which refer exclusively to pro- 

 perties of the sether. Finally, in the Philosophical Magazine 

 for May of the present year, I have discussed several points 

 which, on a review of the arguments employed in the explana- 

 tions of phenomena of both classes, appeared to require correc- 

 tion or confirmation. There was no occasion to indicate parti- 

 cularly the application of the hydrodynamical theorems to cases 

 of diffraction, because the usual theoretical treatment of these 

 phenomena accords well with the characteristics, as determined 

 by my analysis, of the undulations of a continuous elastic fluid. 

 I consider, therefore, that I have now sufficiently exhibited the 

 principles of the application of hydrodynamics in the Undula- 

 tory Theory of Light. 



But the propositions of hydrodynamics are, I maintain, re- 

 quired for ascertaining the laws of the other physical forces, as 

 well as those of light. 1 have, in fact, applied them to some extent 

 in theoretical investigations relative to heat, electricity, galvanism, 

 magnetism, and gravity. With the view of carrying on these re- 

 searches further, I recently submitted to another revision the prin- 

 ciples on which the hydrodynamical propositions rest, and the 

 processes of reasoning employed in proving them ; and the object 

 of the present communication is to give the results of this new 

 inquiry. In treating of the dynamics of undulations, so far as 

 relates to phenomena of light, it is only necessary to consider 

 by what action they cause vibrations of the atoms of sensible 

 bodies, or, conversely, the undulations which the vibrations of the 

 atoms produce in the sether. But in order, on the same prin- 

 ciples, to account for the other physical forces, it is required to 

 investigate the dynamical action of the sether in producing not 

 only vibrations, but also permanent motions of translation, of 

 atoms. My previous researches pointed to two different modes 

 in which the latter kind of motion might be impressed on 

 spherical atoms. In the one the motions result from pressures 

 of the sether accompanying its vibrations ; in the other, from 

 pressures accompanying its steady motions — that is, motions 

 which are functions of space but not of time. The forces of 

 heat and of aggregation, and the force of gravity, are, accord- 



