2 61: Proceedings of the Royal Fhysical 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 etlier^ 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, 
dj^Vj dP"Uj vi/w -rr/^"-^ T^/ ' x^/ 
HF ^ + ^ 
p, d^ u 
dy'^ 
+ H' 
\d''u 
d?u 
d'u^ 
^dx'' 
dy^ + 
dz"- ] 
d'^v' 
^ d^ 
+ IF 
d^v' 
dz^ 
dh> 
dx^ 
d'v 
d'v 
dz'' ] 
w' 
dz'^ 
d"w 
dx^ 
d'w 
d'w 
d^ 
^ ^ ^ d^ + ^ + P 1^ + P 
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 
IV 
a 
- J - 
7 
'id 
v' 
iv' 
a 
~ w ~ 
7 
^ {Ix + my + nz — vt} 
. 2. , \ (23) 
= —r {tx + my -\- nz — vt} ) 
On making these substitutions each of the equations 
(21) gives the same equation of condition, in order that the 
above values oi u v w, u' v' w may satisfy them. 
Each of the equations (22) also gives the same equation 
ol' condition. 
