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Photo from Bureau of Standards 



A MARVEIv OF PRECISION : THIS RI]?FLE;r CLOCK 



This is the most accurate type of clock made and will keep time to within a variation 

 of half a second a month. Its pendulum is of nickel steel, so that the expansion is negligible. 

 It is self-winding and runs in a partial vacuum. It is regulated by admitting more or les. 

 air into the partial vacuum, and it is kept in a constant temperature vault, every precaution 

 being taken to free the atmosphere from moisture. It is checked every day by observations 

 taken at the U. S. Naval Observatory. 



159 



