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When we were describing the motion of a material point relative

to a body of reference, we stated nothing more than the

encounters of this point with particular points of the

reference-body. We can also determine the corresponding values of

the time by the observation of encounters of the body with

clocks, in conjunction with the observation of the encounter of

the hands of clocks with particular points on the dials. It is

just the same in the case of space-measurements by means of

measuring-rods, as a little consideration will show.