12 F. L. Ekman, 



in the form of rain and partly in that of melting ice, and another not 

 inconsiderable quantity falling on a narrow belt in the vicinity of the 

 equator, the two intermediate zones, from near the equator to about 

 40" latitude, being comparativel}^ but poorly supplied with rain. The ge- 

 neral result of rain at sea then, separately considered, ought to be four 

 surface-currents debouching upon the zones ill supplied with rain: two 

 greater, carrying cold and mixed sea-water from the poles to the tro- 

 pics, deviating towards the eastern, and two smaller, from the equator 

 to the tropics, deviating towards the western coasts. 



An other and by no means inconsiderable amoimt of rainfall is 

 carried into the sea by the rivers, the mouths of which form the com- 

 mencements of currents, which however usually have onl}^ a local im- 

 portance. In any places, e. g. in the Skagerack, where the collected 

 mass of several rivers falls in at one and the same creek, the quantity 

 of fresh water can nevertheless be sufficient to give rise to streams of 

 such strength and extent, that they must be attended to in navigation, 

 and even in the question of the general .oceanic circulation the influence, 

 which the outpourings of the rivers may exert on the waters motion 

 must not be entirely left out of consideration. 



Ill: O Streams caused by evajjoraiion. 



In fresh water evaporation causes a sinking of the surface and a 

 cooling of the external stratum. When sea-water evaporates, its saltness 

 is also enhanced and accordingly its specitic weight increased from two 

 causes, producing, as Maury has already shown, a vertical circulation 

 within the evaporating locality. This vertical circulation however soon 

 reaches a limit, beneath which it cannot easily extend. In those regions 

 of the ocean, where evaporation is greatest, the solar heat operates most 

 powerfully; thus the increasing of the specific gravity is counteracted 

 by the heat, and, by other reasons, strata, that are heavy in conse- 

 quence of lower temperature, keep there constantly rising to the surface. 

 Again in localities, where the surface daily loses more heat than it re- 

 ceives, the evaporation is little compared with the accensions of fresh 

 water, and the sinking surface-water therefore soon meets with strata 

 of sufficient saltness and weight to render the circulation difficult. 



Of much greater importance in the oceanic circulation is the 

 change of level produced b)^ evaporation; for the increased hydrostatic 

 pressure, arising from the increased specific weight, is vanishingly small 



