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When is a Watch Keeping Good Time?
Part Two
  ... Naval Observatory in Washington whose time signals are the basis for airline, railroad, radio and television scheduling.  Accuracy is something else to a citizen deciding what time to leave for the office.  At the Naval Observatory, time is measured with the world's finest clocks.  For many years pendulum clocks were used as primary standards.  They performed with excellent precision and the last ones used - the Shortt Clocks - had rates constant to aboue one hundredth of a second a day.  To protect these clocks against moisture, jarring and temperature change, a special vault was constructed in the early 1900's with a ceiling of rock wool, walls of brick and a cement floor.  Both temperautre and humidity in the vault were kept constant.  The development of the quartz crystal clocks and their adoption as primary standards in 1946 signaled the end of the pendulum era.  The oscillating quartz crystal varies less than one ten-thousandths of a second a day.  But even this has been surpassed by clocks used as today's primary standards.  Accurate as these clocks are, the are checked daily against carefully made star observations.  Thus the relative accuracy of any man-made timekeeper can be ....  
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