26-1 Proceedings of the Royal Physical Society. 



molecules of the solid, and by the forces exerted upon them 

 by the etherial medium. 



On this supposition, the equations of motion of the ma- 

 terial molecules may be immediately written down by analogy 

 from the equations of motion of the ether. 



For if we substitute matter for ether t and ether for matter, 

 the above investigation of the equations of motion of the 

 ether applies also to this case, and we are thus enabled to 

 write down at once the equations of motion of the material 

 molecules. 



They are, therefore, 



d 2 u' TV d 2 u' ' d 2 u TT/ d 2 u , 



dF = 1 d^ + G ^ + H ^ ~ Pu + Pm 



d 2 u d 2 u d 2 u 

 + 



dx 2 dy 2 dz 



d 2 v' ™ d?v' ~, d 2 v' tt / d 2 v' , 

 W ~ dx^ + dt/ 1 + dz 7 ^ + 

 f d 2 v d 2 v d 2 v 1 



■f 1 { -T~ 0 + — o + 



t dx 2 dy 2 dz 



d 2 w' -n, e£ 2 w' ~, d 2 w TT/ d 2 it/ „. , -rt, 



+ F 



All the constants in these equations must in general be 

 supposed different from the constants in equations (21). In 

 order to investigate the motion of plane waves, let us put 



u v id . 2v r7 > 

 — = — - = — = sin — \ lx + my + nz — vt\ » 

 a 0 7 X 1 J J i 



^ (23) 



—. = -ztt = —r — sm — — \ lx + my -t- %2 — v£j l 

 a |S 7 X * J ' 



On making these substitutions each of the equations 

 (21) gives the same equation of condition, in order that the 

 above values oiuv w, u' v' id may satisfy them. 



Each of the equations (22) also gives the same equation 

 of condition. 



