To the philosopher, time is one of the fundamental quantities. To the average man, time has something to do with dinner. Both find it incapable of definition but subject to measurement, and to the latter at least, the measurement is of importance. Moreover, the importance of the measurement and correspondingly of its accuracy has increased as the complexity of life has increased, so that he no longer finds an estimate from the sun's position satisfactory. Instead he not only wants but needs a portable device for measuring time intervals, which device he calls a watch and upon whose accuracy he sets requirements which are astounding when compared to the accuracy of other instruments.

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