1870.] 



Deep-sea Researches. 



213 



loss by evaporation from its surface is scarcely at all replaced by fresh water 

 either from rain or rivers. 



123. Now if it can be shown that a similar vertical circulation is main- 

 tained in the opposite direction, when the conditions of the case are altogether 

 reversed, the explanation above given may, it is submitted, be regarded 

 as having a valid title to acceptance. Such a converse case is presented 

 by the Baltic, an inland basin which communicates with the German Ocean 

 by three channels — the Sound, the Great Belt, and the Little Belt — ot 

 which the Sound is the principal. The amount of fresh water discharged 

 into the Baltic is largely in excess of the quantity lost from its surface by 

 evaporation ; and thus its level would be continually raised, if it were not 

 kept down by a constant surface -current, which passes outwards through 

 the channels just mentioned. But the influx of fresh water reduces the 

 density of the Baltic water ; and as the water which the outward current 

 is continually carrying off contains a large quantity of salt, there would be 

 a progressive reduction of that density, so that the basin would at last come 

 to be filled with fresh water, if it were not for a deeper inflow. Such an 

 inflow of denser water might be predicted on Principle VI. as a Physical 

 necessity, arising from the constant want of equilibrium between the lighter 

 column at the Baltic end of the Sound and the heavier column at its out- 

 let in the German Ocean ; and that such an undercurrent into the Baltic 

 has an actual existence, was proved two hundred years ago by an experiment 

 of the same kind as that by which we have recently proved the existence 

 of an undercurrent out of the Mediterranean. This experiment is cited by 

 Dr. Smith (loc. cit.) in his discussion of the Gibraltar Current, as supply- 

 ing an analogical argument for his hypothesis of the existence of an under- 

 current in the Strait of Gibraltar ; but he does not make any attempt to 

 assign a Physical cause for the movement in either case*. — The condition of 

 the Euxine is precisely parallel to that of the Baltic ; and a surface-current 

 is well known to be constantly flowing outwards through the Bosphorus 

 and the Dardanelles, carrying with it (as in the case of the Baltic) a large 

 quantity of salt. Now as the enormous volume of fresh water discharged 

 into the Euxine by the Danube, the Dnieper, and the Don would in time 

 wash the whole of the salt out of its basin, it is obvious that its density can 

 only be maintained at its constant amount (about two-fifths that of ordinary 

 sea-water) by a continual inflow of denser water from the iEgean, — the 

 existence of which inflow, therefore, may be predicted on the double ground 

 of a priori and a posteriori necessity. 



General Oceanic Circulation. 



124. The difference as to Level and Density between two bodies of sea- 

 water, which produces the vertical circulation in the Strait of Gibraltar 



* Prof. Forchhammer fully confirms Dr. Smith's statement ; and further shows that 

 the water which thus returns to the Baltic has the density of Sound water, the surfaces 

 current being formed of the much lighter Baltic water. 



