34 TIDES. 



place — which is twenty-four hours, fifty minutes and a half — 

 the sea ebbs and flows twice, much less, indeed, towards the 

 poles than within the tropics, where the waters lie under 

 the direct influence of the lunar attraction. It is in the 

 southern hemisphere that the tidal wave originates, and 

 from thence moves northward, influenced in its direction by 

 the motion of the earth. Almost excluded from the North- 

 ern Pacific by the barrier of islands and coral reefs which 

 stretch across from Australia nearly to South America, the 

 effect of the tides, excepting on the west coast of that con- 

 tinent, is little felt in that ocean. In the Indian Ocean, 

 compressed between Africa on the north and Australia and 

 Sumatra on the east, it bursts in full strength on the shores 

 of Hindoostan. In the narrow channel of the Atlantic the 

 tidal wave progresses northward with great rapidity, and on 

 the shores both of Europe and America, producing, as in 

 Southern India, the Bore, which is described in the chapter 

 on the " Phenomena of the Ocean," 



The highest floods and the lowest ebbs occur at the 

 period of new and full moon, near the equinoxes, in March 

 and September, when the moon is nearest the earth. 



Winds have also a powerful influence over the tidal cur- 

 rents, especially in narrow seas, keeping them back when 

 blowing from an opposite quarter, and quickening their flow 

 when pursuing the same direction ; but the motion of the 

 water in the tide-wave is totally unlike that in an ordinary 

 surface-wave, such as the wind produces ; and it differs, also, 

 in affecting the ^vhole depth of the ocean equally from the 

 bottom to the surface, while the wind-waves, even in the 

 most violent storms, agitate it to a very trifling depth. In 

 the deep water of the ocean, the tidal-wave does not exceed 

 twelve feet in height. 



The ancients knew that the time of high water, and also 

 the height of the tide, were in some way connected with the 

 age of the moon. It was the illustrious Sir Isaac Newton 



