THE ELECTRIC AND LUMINIFEROUS J\1EDIUM. 733 
media, to avoid static instability; that condition is in fact required as below, in order 
that waves may be transmissible at all through a heterogeneous medium of this type. 
As an illustration of this some^rhat abstract discussion, let us conduct the variation 
of the Action in this labile elastic-solid medium. The equation takes the form 
+ 
W dyl 
fciv 7 ' 
\ chj dz dz dx dx dyj] 
= 0; 
it would be illegitimate for the present purpose to replace the potential energy by a 
surface part and a volume part, because then it would not be correctly located in the 
medium. We obtain on the left-hand side the time-integral of the expression 
- "f {Cf + S) + nSv) + (f + :f j («Sf + klQ 
+ (x + ‘H) dh + m'd) -2(^ + 5) - 2 (f + f) 
,dx dy 
dr] 
^ [dx dy 
nhl,\ c/S 
+ 
" f i) (i s + + * 
+ (S’) j, + sf 
d . o d\[dri , d^\ ^ d. fdrj , r/f ' 
dyj \dx 
dl 
dx 
+ 
dy 
- 2 3 £- (-r + ; 
dx \dy dz 
or collecting and exhibiting specimen terms only. 
The equations of motion are thus 
