761 
a consequence of which is 
Wha = — Was 
and we shall complete our definition by ') 
te UF Ns) LSS SS. AP ON eed 
The term we are considering then becomes 
Beer Og Oda A a ein 10 08 
= (ab) Wars’ Ee Ee == an War (e-3<)= 
Oapy 
0 09a bein, 
= 4 2 (ab) val Te ) = (ab) Was SE = 
ee > (a b) an qa) ii > (a b) = ae 3 
so that, using (14), we el for a 
= + (a) OWas 
~ ga + 
JL + & (a) Ka Ova = — & (ab) 
+ & (ab) ia Wi Mie A (a) Keita se rs) 
where we have taken into consideration that 
= (ab) Wap (wy Ova — Wa Ory) = & (ab) Wad Wh Ow. 
If we multiply (40) by dS and integrate over the space S the 
first term on the right hand side vanishes. Therefore (12) requires 
that in the subsequent terms the coefficient of each qq and of each 
Ova be 0. Therefore 
Ow; 
EO =O ee (4) 
vh 
and 9 
LE A ENE De NRO ena 
by which (40) becomes 
dL ne = oie : 
+ 3(a) Ka Ora = — > (ab) (43) 
In (41) we have the second set of four AED equations, 
while (42) determines the forces exerted by the field on the charges. 
We see that (42) agrees with (19) (namely in virtue of (31). 
§ 12. To deduce also the equations for the momenta and the 
energy we proceed as in $ 6. Leaving the gravitation field unchanged 
we shift the electromagnetic field, i.e. the values of wa and wa, 
in the direction of one of the coordinates, say of w,, over a distance 
defined by the constant variation Ov, so that we have 
1) The quantities Yap are connected with the quantities g%, introduced by 
Einstein by the equation Was = Vg. Pab: 
