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In view of the resuIts of these considerations we are led to the

conviction that, according to the general principle of

relativity, the space-time continuum cannot be regarded as a

Euclidean one, but that here we have the general case,

corresponding to the marble slab with local variations of

temperature, and with which we made acquaintance as an example of

a two-dimensional continuum. Just as it was there impossible to

construct a Cartesian co-ordinate system from equal rods, so here

it is impossible to build up a system (reference-body) from rigid

bodies and clocks, which shall be of such a nature that

measuring-rods and clocks, arranged rigidly with respect to one

another, shall indicate position and time directly. Such was the

essence of the difficulty with which we were confronted in

Section XXIII.