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National Geographic : 1947 Sep
Contents
Split-second Time Runs Today's World Hamilton Watch Company This Tiny Hairspring Controls the "Heartbeat" of a Watch Vibrating 18,000 times an hour, it governs the swing of the balance wheel in a lady's watch. The balance wheel in turn controls the action of the mainspring on the intricate cogwheel mechanism. Less than 3 the diameter of an average human hair, the hairspring is made of rustproof Elinvar, a nickel-steel alloy. drumhead, or the air column in the tube of a trumpet or tuba. The more vibrations per second, the higher the pitch. To help musicians and musical instrument manufacturers tune their instruments accu rately, the Bureau of Standards broadcasts a continuous musical note precisely tuned to A above middle C. It is produced by dividing down the vibrations of a crystal oscillating at 100,000 cycles per second to 440 cycles per second. This note is the Nation's "standard of pitch." You can get it any time on a short wave radio and use it to tune your piano, violin, or trombone. Government surveys, which form the basis for all property lines, depend on very accurate measurement of latitude and longitude. To measure longitude you need to know the cor rect time to a small fraction of a second. Longitude is distance measured east or west of the Greenwich meridian, an imaginary line that runs through the original site of Britain's Royal Observatory at Greenwich. (The Observatory is now being moved to Sussex because smoke of factories made ob servations difficult.) Distance from the Greenwich meridian is measured not in miles but in time-in hours, minutes, seconds, and fractions of seconds. To get it, you have to know the exact time on the Greenwich meridian and the exact time where you are. The difference between the two is your longitude. For example, if it is 12 noon in Greenwich and 7 a. m. at New York City, the longitude of New York is 5 hours west. On maps, longitude is indicated in degrees, one hour being equivalent to 15 degrees. Government surveys are made to an accuracy of 1/100 of a second in longitude. Without a chronometer, which is really a clock that tells very accurate time, no ship could navigate accurately. From the chronometer the ship's navigator can determine the correct time at Greenwich. Then by observing the positions of certain stars and checking with official navigation tables he can easily determine his ship's posi tion (pages 408 and 428). Congress Turned Time Backward Congress sometimes used to turn time back ward in order to transact its business "on time." Formerly, when sessions ended at mid night on March 4, by the clock in the Senate 407
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