where we must give the values 1, 2, 3 to and .
The gravitation energy lying within a closed surface consists therefore of two parts, the first of which is
while the second can be represented by surface integrals. If namely are the direction constants of the normal drawn outward
In the case of the infinitely feeble gravitation field represented by (§ 57) both expressions and contain quantities of the first order, but it can easily be verified that these cancel each other in the sum, so that, as we knew already, the total energy is of the second order.
From and the equations of § 32 we find namely
so that we can write
The factor is of the first order. Thus, if we confine ourselves to that order, we may take for all the other quantities these normal values. Many of these are zero and we find
Here we must take ; , while we remark that for the expression between brackets vanishes. For the integral becomes do, which after summation with respect to gives
representing the normal to the surface. If and differ from each other, while neither of them is equal to 4, we can deduce from (110) and (109)