100 



THE OCEAN. 



same coinciderce for the semi-monthly period ; the return of the 

 spring-tides occurs from fortnight to fortnight with the return of 

 the full or new moon, and the monthly period is completed when the 

 series of lunar phases recommences. Kor is this all ; the tides have 



^S=E=^=fc 





Ti vn Tox n 



X XI Noon. I II m IV V 

 Fig. so.— Tide at Southampton, 2nd August, 1869. 



TI "VII Via 



also their semi-annual period, from the equinox of March to that of 

 September, for the sun being then directly above the terrestrial 

 equator, exercises a stronger attraction on the liquid masses, and the 

 waves of the spring- tides rise to a greater height than usual. 

 Finally, an annual period is marked for the tides by the epoch when 

 the earth is nearest the sun. This epoch falls during the winter of 

 the northern hemisphere, and it is then indeed that the spring-tides 

 rise with most force on the coasts of our continents. 



Thus the phenomena of the tides are intimately connected with the 

 celestial movements, and every change in the relative position of the 

 bodies which attract our planet, manifests itself by a corresponding 

 change in the level of the seas. Knowing beforehand the route which 

 the earth follows in space, astronomers foresee thereby even the future 

 oscillations of the wave, and can trace their curve for centuries to come. 

 Nevertheless, it must be admitted, this curve is only true in theory ; 

 for if the tides in their origin be due to astronomical causes, they are 

 also subject to variations from terrestrial phenomena. Like the winds, 

 currents, and all the other manifestations of planetary life, they pre- 

 sent incessant variations, and are, so to say, in a continual genesis. 



