OCEANOGRAPHY. 15 



Again, the ocean water itself is not of the same 

 density in all latitudes. In regions where there 

 is a copious rainfall and the sea is not frequently 

 disturbed by severe storms, the rain takes some 

 time to mix with the heavier salt water on which 

 it falls, and consequently there may always be- 

 discovered in these localities a thin stratum of 

 comparatively fresh water on the surface of the 

 ocean. In some inland seas where there is con- 

 siderable evaporation and a slight rainfall, as for 

 example, the Eed Sea and the Mediterranean, the 

 sea-water reaches an even higher degree of con- 

 centration than it does in the open ocean. The 

 following table will serve to illustrate these 

 facts : 



Density of rain-water, 1 '00. 

 Density of the Black Sea surface, below 1 -025. 

 Density of the Atlantic Ocean surface (west of 



the Canaries), 1-0275. 



Density of the Mediterranean Sea, over 1-028. 

 Density of the Red Sea, 1 '030. 

 Density of the bottom water of the Atlantic, 

 1-029 (west of the Canaries). 



That the rate of movement of the water influ- 

 ences very largely the character of the animals 

 that live in it, is a fact that it is not necessary to 

 discuss fully in this place ; but as it is undoubtedly 

 one of the factors which must be taken into con- 

 sideration in discussing the character and pos- 

 sible origin of the Fauna of any particular region, 

 a brief survey must be given of some of the 

 principal causes of the movements of the water 

 and the characters of the tides and currents which 

 are manifest in the sea. Twice every twenty- 



