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This stipulation contains a further physical hypothesis, the

validity of which will hardly be doubted without empirical

evidence to the contrary. It has been assumed that all these

clocks _go at the same rate_ if they are of identical

construction. Stated more exactly: When two clocks arranged at

rest in different places of a reference-body are set in such a

manner that a particular position of the pointers of the one

clock is simultaneous (in the above sense) with the same

position, of the pointers of the other clock, then identical

"settings" are always simultaneous (in the sense of the above

definition).

IX.

THE RELATIVITY OF SIMULTANEITY