ha Pacs 23 
5 (23) 
we tind 
OL OP S 
AN EN se a nk 
Oa w 024 
The first three of these equations (ce = 1, 2,3) refer to the momenta; 
the fourth (ce = 4) is the equation of energy. As we know already 
the meaning of ,,....K, we can easily see that of the other 
quantities. The stresses. AG, A, Xe Nave. are Ly, ared ars dns oyu 
the components of the momentum per unit of volume —7’,,, —7’,,, 
—T’,,; the components of the flow of energy 7, 7, 7, Further 
T,, is the energy per unit of volume. The quantities 
se & OL 
are the momenta which the gravitation field imparts to the material 
system per unit of time and unit of volume, while the energy 
OL 
which the system draws from that field is given by -— (; ) 
U, w 
An electromagnetic system in the gravitation field. 
§ 7. We shall now consider charges moving under the influence 
of external forces in a gravitation field; we shall determine the 
motion of these charges and the electromagnetic field belonging to 
them. The density @ of the charge will be supposed to be a con- 
tinuous function of the coordinates; the force per unit of volume 
will be denoted by A and the velocity of the point of a charge by 
v. Further we shall again introduce the notation (10). 
In Einsteins theory the electromagnetic field is determined by 
two sets, each of four equations, corresponding to well known 
equations in the theory of electrons. We shall consider one of these 
sets as the mathematical description of the system to which we have 
to apply Hamiton’s principle; the second set will be found by 
means of this application. 
The first set, the fundamental equations, may be written in 
the form 
d Wa b 
ae ee Way . 
Oe, 
>) (25) 
