in the Air and in the Sea. 35 



sure prevails. It is self-evident that a current thus produced 

 in the ocean can only be very inconsiderable and inconstant, 

 since it changes with every change of the atmospheric pressure. 

 This force, again, can only make itself perceptible in channels. 



Suppose that on an inland sea connected with the ocean 

 only by a strait the barometer suddenly fell 1 inch, the level of 

 this sea would stand 13*6 inches lower than equilibrium with 

 the ocean at the moment would require. The mass of water 

 wanting must therefore pass from the ocean through the strait. 



"We must ascribe it to this circumstance that, in the Sound, a 

 change in the direction of the current mostly occurs 24 hours 

 before a change in the direction of the wind. Just so the water 

 usually rises in the Gulf of Finland before the south-west wind 

 commences; and this rise is noticed also in winter, when the 

 entire surface of the sea is covered with ice, and thereby the 

 direct action of the wind is withdrawn. 



b. Saltness of Sea-water. 



Alterations in the saltness of seas greatly affect the specific 

 gravity of the water. The observations, however, which have 

 been made in different parts of the world have proved that the 

 difference in saltness of the various oceans is extremely slight*. 

 This compels us to the conclusion that currents immediately 

 tend to equalize the slightest difference in the saltness of the 

 water. 



The causes of change in the saltness may be accidental and 

 temporary, or constantly repeating themselves in certain regions. 

 In the first case they produce variable currents, which do not 

 belong to the subject we are considering; but in the second 

 they must confer upon the water a constant tendency to inter- 

 change, and call forth constant currents. 



In the equatorial regions, for example, the half inch of water 

 evaporated daily leaves constantly its salt behind, which, with 

 the vast depth of the ocean, can hardly add perceptibly to the 

 specific gravity of the rest of the water. 



Nevertheless this water, very gradually becoming slightly 

 Salter and heavier, and sinking, must occasion in the depths a 

 current, although a very feeble one, the direction of which must 

 be into the regions where there is little evaporation and consider- 

 able atmospheric precipitation, therefore into higher latitudes. 

 Consequently, as we have already seen., the current called forth 

 by the evaporation of the water counteracts that which is pro- 

 duced by the expansion of the water from difference of tem- 

 perature. 



* " On the Composition of Sea-water in different parts of the Ocean," 

 Phil. Trans. Roy. Soc. London, 1865, p. 203. 



D2 



