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Fourier Analysis of Ocean Tides I

A typical tide record

From George H. Darwin's article "Tides" in the 9th edition of the Encyclopaedia Britannica. Note that in this graph, time increases to the left.

This figure shows the tidal record for two weeks (January 1-14, 1884) at Bombay. The tide was recorded on a cylindrical sheet that turned once every 24 hours. Each daily curve is labelled with its date. Some obvious features:

  • there are usually two high tides and two low tides each day;
  • the tides come about 50 minutes later each day;
  • during the two-week period there is considerable variation in the daily pattern of highs and lows;
  • there is usually a difference in height between two consecutive high tides (the diurnal inequality).