765 
dL Ors, 
—( )=-= 05; er cig emmy > 
L Ot, 
Remembering what has been said in § 12 about the meaning of 
mi] 
dL 
& , we may now conclude that the quantities 7, have the same 
ve LZ 
meaning for the gravitation field as the quantities Ts for the electro- 
magnetic field (stresses, momenta ete). The index g denotes that TT"; 
belongs to the gravitation field. 
If we add to (53) the equations (44), after having replaced in them 
b by e, we obtain 
OT 
Kz ZO dt ete Seele Be (OA 
where 
mt rp 
f= Dee + Feen 
The quantities dijn represent the total stresses etc. existing in the 
system, and equations (54) show that in the absence of external 
forces the resulting momentum and the total energy will remain 
constant. 
We could have found directly equations (54) by applying formula 
(50) to the case of a common virtual displacement Ox, imparted 
both to the electromagnetic system and to the gravitation field. 
Finally the differential equations of the gravitation field and the 
formulae derived from them will be quite conform to those given 
by Einstein, if in Q we substitute for H the function he has chosen. 
§ 16. The equations that have been deduced here, though mostly 
of a different form, correspond to those of Einstein. As to the cova- 
riancy, it exists in the case of equations (18), (24), (41), (42) and (44) 
for any change of coordinates. We can be sure of it because LdS 
is an invariant. 
On the contrary the formulae (49), (53) and (54) have this pro- 
perty only when we confine ourselves to the systems of coordinates 
adapted to the gravitation field, which Einsrrin has recently con- 
sidered. For these the covariancy of the formulae in question is a 
consequence of the invariancy of 0/ HdS which Einsrrix has proved 
by an ingenious mode of reasoning. 
