336 GEOLOGY. 



are referred, it is to be remembered that the sea has ''a very compli- 

 cated undulating surface in consequence of the attraction which the 

 heterogeneous and elevated portions of the hthosphere exercise on the 

 Hquid hydrosphere. In the opinion of geodesists, the geoid may in 

 some places depart from the figure of the spheroid by 1000 feet/^ ^ 

 These variations in level would, however, not occasion circulation. 

 The differences in level which determine circulation are much more 

 trivial. Every stream which pours fresh water into the sea tends to 

 raise the level of the water where it enters. The waters brought to the 

 ocean by the Amazon, the Mississippi, and other great rivers would ap- 

 preciably change the level of the sea at their debouchures, if the excess 

 did not promptly flow away. The ready mobility of the water, however, 

 prevents its accumulation, and the discharge of every stream generates 

 widespread movement. This movement is strongest at the debouchure, 

 and weakens with increasing distance from it, though in the case of great 

 streams, such as the Amazon, the movement is traceable, by means of 

 the sediment which the water carries, hundreds of miles out to sea. 



Changes of level are also brought about by the winds, which pile up 

 water along the shore against which they blow. The level of the water is 

 said to have risen 24 feet at Calcutta on October 5, 1864, as the result 

 of a severe storm. While this is exceptional, a rise of 2 feet is not rare. 

 This piling up of the waters along shore insures a compensating move- 

 ment (undertow, Httoral currents, etc.) in some other direction. Unequal 

 evaporation and precipitation likewise disturb the level of the sea and 

 occasion movement. In the open sea the movements generated by 

 differences of level, Hke those generated by differences of density, are 

 chiefly slow, creeping movements, but movements which never cease. 

 In bays and gulfs, on the other hand, the surface of the water may be 

 so raised, either as the result of wind, river discharge, or heavy pre- 

 cipitation, as to give rise to strong outward currents. There is httle 

 doubt at the present time that the Gulf Stream owes its origin primarily 

 to the difference of level between the Gulf of Mexico and the Atlantic. ^ 



Movements generated by winds. — The circulation resulting from the 

 tendency of the winds to change the level of the sea-water has already 

 been mentioned, but the wind also works in other ways. Where the 

 winds have a somewhat constant direction and are at the same time 



^Murray. Scottish Geographical Magazine, Vol. XV, p. 507. 

 'Lindenkohl. Science, Vol. X, 1899, p. 807. 



