will then be infinitely small quantities of the first order, and
will equal 1, except for quantities of the second order. From the equations defining the
it will then be seen that these quantities will be infinitely small, of the first order. If we neglect quantities of the second order, and finally assume that, for small velocities of matter, the changes of the gravitational field with respect to time are small (i.e. that the derivatives of the s with respect to time may be neglected in comparison with the derivatives taken with regard to the space-co-ordinates) the system of equations (1) assumes the form:
This would be the equation of motion of a point-mass as already given by Newton's mechanics, if
be taken as representing the ordinary gravitational potential. It still remains to be seen what the differential equation for
