1871.] 



Theory of the Ocean. 



539 



conclusions to be drawn from the facts he gives regarding the densities and 

 currents, and thus led Dr. Carpenter to adopt them as follows: — 



" Now if it can be shown that a similar vertical circulation is maintained 

 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, of 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 evapora- 

 tion; 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 outlet in the German 

 Ocean ; and that such au 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 dis- 

 cussion of the Gibraltar current, as supplying an analogical argument for his 

 hypothesis of the existence of an undercurrent in the Strait of Gibraltar ; 

 but he does not make any attempt to assign a physical cause for the move- 

 ment in either case"*. — Proceedings of Royal Society, vol. xix. p. 213. 



I need only add, as a remark to this latter part, that a greater density 

 below was no proof of an undercurrent, as shown by the contrary fact in the 

 Dardanelles and Sea of Marmora. 



In his paper in the Philosophical Transactions for 1865, "On the Com- 

 position of Sea-water in the different parts of the Ocean," in page 230 

 Dr. Forchhammer says, " In the Baltic likewise the water from the deeps 

 contains more salt than that from the surface. The upper-current goes 

 generally (not always) out of the Baltic, the reverse of the Mediterranean. 

 The cause is evident, the excess of atmospheric water in the Baltic from 



the rivers surrounding it With the assistance of Captain Prosilius, in 



1846, who commanded the vessel stationed at Elsinore, the surface-current 

 was observed on 134 days, from 2/th April to 11th September; of which 

 on 24 days it ran from the north, on 86 from the south, and on 24 days 

 there was no surface-current at all." 



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

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

 surface-current being formed of the much lighter Baltic water." 



