Baron N. Schilling on the Constant Currents 



•a 



In the polar sea much water is, in winter, turned into ice, 

 and its salt separated. This salt adds, though only inconsider- 

 ably, to the specific gravity of the cold, and hence already heavy, 

 polar water, and contributes a little to the undercurrent in the 

 direction of the equator ; so that it acts, contrary to the pre- 

 ceding case, just the same as the difference of temperature. 

 Perhaps it is partly owing to this, that the flow of the Gulf- 

 stream is somewhat stronger in winter than in summer. 



In an extremely interesting article on the currents at the 

 southern extremity of America, Miihry*, on the ground of the 

 winter temperature of Patagonia, conjectures that the Brazilian 

 current also is stronger in the winter of the southern hemi- 

 sphere than in summer. 



In summer, when the polar ice melts, a superficial polar 

 current results; for the water from the melting of the ice, 

 having but little saltness, remains at the surface in spite of its 

 coldness. Scoresby remarked that near Spitzbergen the water 

 of the surface was warmer than at some feet depth ; and this 

 observation has been recently confirmed by the Swedish Expe- 

 dition and by the Norwegian Captain Ulvef. 



Unfortunately, we do not yet possess any accurate determina- 

 tions of the greatest density of sea-water at different tempera- 

 tures and under various pressures. At all events, however, 

 Miihry's view, that sea-water, as well as fresh water, attains its 

 greatest density at + 4° C, appears destitute of proof; for it 

 has recently been found that the temperature at very great 

 depths is often below 0° C. It is probable that the great pres- 

 sure to which the water is there exposed has an influence in 

 preventing it from freezing, even below 0° C. If earlier obser- 

 vations seem to contradict this, the reason may well be that the 

 thermometers were not sufficiently protected against the pres- 

 sure of the depths, and hence always gave the temperature of 

 the bottom too high. On this defect rests also Boss's well- 

 known theory of a constant temperature of +39° F. at the 

 bottom of the sea. 



At the mouths of rivers, the fresh water must spread upon 

 the surface, and therefore give rise to a current running out- 

 wards from the mouth, until it is mixed sufficiently with the 

 salt water. To this end, however, the heavy salt water will 

 flow in the opposite direction, toward the mouth — which, even 

 with a slight current, must greatly favour the formation of 

 sandbanks there. 



In the case of inland seas where the inflow is greater than the 

 evaporation, as e. g. in the Black Sea and the Baltic, just as 



* Peterraann's Geogr aphis che MittJieilungen, 1872, vol. xviii. p. 126. 

 t Ibid. p. 317. 



