On the general Causes of the Ocean-Currents. 33 



cific weight are balanced by differences of level, and the under-current 

 into the Baltic driven only by the reaction of the outflowing surface- 

 current, the occurrence of a great fall of the barometer over the North- 

 Sea, or a great rise over the Baltic, might cause the under-current in 

 the Sound to be reversed in consequence of the hydrostatic pressure in 

 the Baltic being increased down to the bottom. A number of complica- 

 ted cases are here possible, for example lastly among others, that by a 

 rising of the Kattegat in consequence of the effect of the wind in the 

 North-Sea, the Kattegat just arrived at the level of the Baltic and the 

 bottom-water streamed into the Baltic only in consequence of its greater 

 specific gravity '). 



b) Reaction- Streams arising from evaporation. 



When the water in an ocean-basin evaporates more rapidly than 

 the fresh water from the surrounding rivers can compensate, its level 

 must be lowered relatively to that of the ocean; the equilibrium between 

 these water-districts is thus disturbed down to the bottom and the 

 ocean's water flows in. The little additions of hydrostatic pressure, 

 which tlie water in the first named locality experiences in consequence 

 of the salt remaining there, gradually coalesce, and it is clear that, if 

 the water were not changed, the higher water-column between the 

 bottom and the surface at the basin's mouth might be at last balanced 

 by the lower one above the level of that depth at the opposite extre- 

 mity of the basin. The difference of pressure in the lower strata would 

 thus be neutralized, but because the inclination of the surface still re- 

 mains, a stream of less depth would constantly flow to the evaporating 

 locality. But as soon as this state of things took place, or then at 

 least, the in-flowing siirface-stream must react on the subjacent stratum 

 in the mouth, according to the laws above expounded. The surface- 

 current therefore carries into the basin more water than what otherwise 

 should be the consequence of the difference of pressure between the two 

 water-regions, and by the abasement of level, which is caused at the mouth 

 by the streams reaction, the equilibrium there again is disturbed down 

 to the bottom. By this an undercurrent arises, which naturally will be 



*) The account I have given of the streams in these basins, is partly founded 

 only on theoretical grounds; we may however soon expect a more complete experi- 

 mental investigation of these waters, carried out at the expense of the Swedish go- 

 verneiiieiit. 



Nova Acta Eeg. Soc. Sc. Ups Ser. III. 5 



