106 THE OCEAN 



of nearly 200 fathoms during heavy gales from 

 the south-west. Some abnormal waves which 

 have been observed in the ocean are known 

 to be due to earthquakes and volcanic erup- 

 tions. 



Tides. — The term tide is applied to the 

 periodical rising and falling of the water of the 

 ocean caused by the attraction of the sun and 

 moon. Periodical alternations in the direc- 

 tion of the wind, and periodical variations in 

 atmospheric pressure, may give rise to alterna- 

 tions in the level of the sea, but true tides 

 are attributed to astronomical causes. It is 

 supposed that the attraction ©f the sun and 

 moon may affect not only the waters of the 

 ocean but also the solid crust of the earth, 

 producing an alternating change in its shape, 

 but so small as to be difficult of detection. 



Any one living at the seaside must have 

 observed the gradual advance and retirement 

 of the sea about twice in the 24 hours, 

 or to be more exact, twice in 24 hours 50 

 minutes, the average interval between two 

 successive high waters being 12 hours 25 

 minutes. The time of high- water thus changes 

 from day to day, and is evidently related to 

 the position of the moon, which passes the 

 meridian on an average 50 minutes later on 

 each succeeding day. The height to which 

 the water rises varies also from day to day, 



