30 LIFE IN THE SEA [OH. 



the spring tides rise to an unusual height. So there 

 is a semi-annual tidal period, and on this is super- 

 posed a semi-lunar one, and on this again a semi- 

 diurnal period. 



Observation has shown that these rhythms are 

 almost absolutely regular. Variations do indeed 

 occur, for exceptionally strong winds blowing from 

 the land prevent the flood tide from rising so high 

 as it otherwise would, and cause the ebb tide to be 

 lower than is predicted ; while strong winds blowing 

 straight in from the sea have precisely the opposite 

 effect. If the barometer rises one twentieth of an 

 inch the flood tide will be an inch less than it would 

 be if the atmospheric pressure were constant ; and 

 if the barometer falls the reverse effect is produced. 

 But these are only apparent irregularities, for with 

 our present knowledge we cannot exactly foresee the 

 variations in the pressure of the atmosphere, and apart 

 from them almost everything is calculable or predict- 

 able. It is known for two or three years in advance, 

 and for a great number of places all over the world, 

 precisely how high the tide will rise, and when it 

 will be high water every day throughout the year ; 

 and so accurate are these tide-tables that sailors 

 have come to accept them almost as part of the 

 order of nature, thinking little of the wonderful 

 mathematical analysis that has made them possible. 



Ordinary observation shows again that once a 



