764 
of a material and in that of an electromagnetic system we need 
consider only the latter. The conclusions drawn in § 11 evidently 
remain valid, so that we may start from the equation which we 
obtain by adding the new terms to (43). We therefore have 
Owi ga d 
dl + 7% 4 Da)Kateg = — Zed) 4 “Se a5 (, : tons + 
Jab.e 
1 : 
+ (ab) dg, es ——— S(ab. “5 (a Q Jen 5c) ee GEN 
wat x04a x Outre Ogab,e 
When we integrate over S, the tirst two terms on the right hand 
side vanish. In the terms following them the coefficient of each dg,» 
must be 0, so that we find 
0Q spy OO rss òL ae 
das 0. Ve Ee ) Elin gab Tectia oe ( ; ) 
These are the differential equations we sought for. At the same 
time si becomes 
ee Ja) 
ji „00 SK dea=— = (ab) —— i a abe ( go he 
tun (50) 
Odab,e 
§ 15. Finally we can derive from this the equations for the 
momenta and the energy of the gravitation field. For this purpose 
we impart a virtual displacement dv, to this field only (comp. $$ 6 
and 12). Thus we put dr, = 0, qa= O and 
Sab = — Yabe Jac. 
Evidently 
dQ 
JQ = — — dw, 
Ree ae 
and (comp. § 12) 
OL 
EER tr) fe 
cae 
After having substituted these values in equation (50) we can 
OL 
deduce from it the value of (==), 
Ue) 
If we put 
ae 1 =) 
jd = — — ae: eer = (ab) Ògane Yab,c Ô . = (51) 
and Tore ie 
ge 1 | 
i heat = (ab) Gab, NT ee bie al) 
x IJab,e 4 
the result takes the following form 
