We start off again from quite special cases, which we have frequently used before.  Let us consider a space time domain in which no gravitational field exists relative to a reference-body K whose state of motion has been suitably chosen.  K is then a Galileian reference-body as regards the domain considered, and the results of the special theory of relativity hold relative to K. Let us supposse the same domain referred to a second body of reference K1, which is rotating uniformly with respect to K. In order to fix our ideas, we shall imagine K1 to be in the form of a plane circular disc, which rotates uniformly in its own plane about its centre.  An observer who is sitting eccentrically on the disc K1 is sensible of a force which acts outwards in a radial direction, and which would be interpreted as an effect of inertia (centrifugal force) by an observer who was at rest with respect to the original reference-body K. But the observer on the disc may regard his disc as a reference-body which is " at rest " ; on the basis of the general principle of relativity he is justified in doing this.  The force acting on himself, and in fact on all other bodies which are at rest relative to the disc, he regards as the effect of a gravitational field.  Nevertheless, the space-distribution of this gravitational field is of a kind that would not be possible on Newton’s theory of gravitation.* But since the observer believes in the general theory of relativity, this does not disturb him; he is quite in the right when he believes that a general law of gravitation can be formulated- a law which not only explains the motion of the stars correctly, but also the field of force experienced by himself.

The observer performs experiments on his circular disc with clocks and measuring-rods.  In doing so, it is his intention to arrive at exact definitions for the signification of time- and space-data with reference to the circular disc K1, these definitions being based on his observations.  What will be his experience in this enterprise ?

To start with, he places one of two identically constructed clocks at the centre of the circular disc, and the other on the edge of the disc, so that they are at rest relative to it.  We now ask ourselves whether both clocks go at the same rate from the standpoint of the non-rotating Galileian reference-body K. As judged from this body, the clock at the centre of the disc has no velocity, whereas the clock at the edge of the disc is in motion relative to K in consequence of the rotation.  According to a result obtained in Section 12, it follows that the latter clock goes at a rate permanently slower than that of the clock at the centre of the circular disc, i.e. as observed from K. It is obvious that the same effect would be noted by an observer whom we will imagine sitting alongside his clock at the centre of the circular disc.  Thus on our circular disc, or, to make the case more general, in every gravitational field, a