On the general Causes of the Ocean-Currents. 11 



II:o Streams produced from rainfall and other accensions 

 of fresh water. 



Wlien rain-water is poured down u^Don a part of the surface of 

 the sea, the equilibrium between that part and the surrounding ocean is 

 in the first instant disturbed down to the bottom; thence a slow, general 

 motion of the sea-water takes place, whereby the level of the surroun- 

 ding water is slightly raised and that of the rainwater-stratum lowered. 

 But the sinking of the fresh water cannot however proceed farther, than 

 to about the point determined by the specific gravities of fresh and salt 

 water. Another motion is thereby caused namely a superficial current, flowing 

 out from the fresh water-stratum in consequence of the dirterence of 

 pressure in the superficial strata. Should the rain cease, results will 

 ensue precisely analogous to those, which take place between a warmer 

 superficial stratum and the surrounding colder water. But should the 

 rain continue with unabated power, the stratum of fresh water must 

 continue to encrease in thickness, untill the consequent rise of level is 

 sufficient to make a surface-current, that carries off" as much water in a 

 unit of time as the rain yields. In this case it seems to be evident, 

 that, in regard only to the differences of pressure , but a superficial 

 stream should be produced without any under-circulation. 



The superficial currents, arising from meteorological wet, frequently 

 have a great velocity, since the sp. gravities of fresh water and sea 

 water are very different; at 15" C. the relation is as 1:1.027, and at 0° C. 

 as 1:1.0283, if the sea-water contains SVg P- c- salt, whereas the greatest 

 difference of sp. gr., that in the ocean can arise from differences of tempera- 

 ture, is about as 1:1.0043. On the other hand it is easily seen, that, if 

 warmth in the one case and rain in the other caused equal differences 

 of pressure at the heavier waters surface-stratum, the depth, at which 

 equilibrium prevails, must in general be greater in the former case than 

 in the latter; or, in other words, the superficial currents, caused by rain, 

 flow in a shallower bed than those caused by heat, provided the maximum 

 of difference of pressure is equal. But then the diff'erences of pressure 

 are smaller at the subsurface-strata of the shallower stream than at the 

 corresponding strata of the deeper, and the former ought thus to move 

 less rapidly than the latter. The intermixture of salt water from below 

 will of course influence the motion ; to that question I shall return farther on. 



The rainfall at sea, is, as is known, very unequally distributed, 

 the greatest abundance falling to the lot of the higher latitudes, partly 



