(113) |

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

(114) |

while the second can be represented by surface integrals. If namely are the direction constants of the normal drawn outward

(115) |

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

(116) |

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

(117) |

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

(118) |

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)