SEA WATER 235 



were covered uniformly with water down to a depth of 

 fifty-three miles the speed of this wave around its surface 

 would be about 1,000 miles an hour. Owing to the com- 

 parative shallowness of our oceans however it has only 

 about half this speed. 



The surface of our earth is not uniformly covered with 

 water, but possesses great land masses which jut out into the 

 ocean and tend to complicate the results of these tidal 

 forces so that a truly satisfactory theory of the tides has 

 not yet been evolved. It is however safe to say that the 

 primary cause is that given above. 



The only region on the earth where the ocean forms a 

 continuous band around the globe, uninterrupted by land 

 masses, is in the southern ocean and there is a theory that 

 around this belt sweeps the great tidal wave, a " primary 

 wave." This wave is supposed to give off secondary, 

 or " derived waves," which move in a northerly direction 

 up the other oceans. Such a wave moves up the Atlantic 

 Ocean at a speed of about 500 miles an hour (Fig. 49). 

 In the open ocean the wave is only two or three feet in 

 height, so that the tides in oceanic islands are very small. 

 But as the water shallows round the edge of the ocean the 

 speed of the tidal wave is enormously reduced by friction 

 and the height of the tides becomes greater. Thus the 

 height of the tide at the Azores is about rive feet, that 

 around the Scilly Isles sixteen feet, and that at Swansea 

 twenty-seven feet. As the tide runs up narrow channels 

 the water tends to be bottled up and therefore the highest 

 tides are found in such channels as the mouth of the 

 Severn, where the tide may reach nearly as much as fifty- 

 feet at Chepstow. 



There are produced around our coasts two high tides in 

 the twenty-four hours and two low tides, but owing to the 

 fact that the moon is also moving round the earth the 



