On the general Causes of the Ocean-Currents. 19 



a) Iit-act /<i/i-St rediUii in consequence of river-trater. 



Although ill the ocean reaction-streams niay, as we have seen, 

 arise from many causes, I will here in the first place confine myself to 

 those produced by the outflow of rivers, because from the special exam- 

 jiles of such streams, which I have the opportmiity of adducing, the 

 reader will be better able to form an idea of the nature of reaction- 

 streams. Before however passing to these examples, it will be neces- 

 sary to take a somewhat general view of the question of river-water's 

 effect upon the sea. 



It has long been observed that in straits, which unite the ocean 

 with separated oceanic basins, in which the annual influx of river-water 

 exceeds the evaporation from the surface, there is beneath the superfi- 

 cial current, which carries off the superfluous river-water to the sea, an 

 under-current of salt water in the opposite direction. This phœnomenon 

 is in fact of a still more general nature. Every river or brook, which 

 falls into the sea, gives rise to an under-current directed towards its 

 embouchure. These mider-currents are so distinct and the causes, that 

 produce them, so active, that in calm weather their presence may be 

 easily observed at the mouth of the most insignificant rivulet, that falls 

 out over the surface of the sea. 



It has been usual to explain the under-current in such straits, as 

 e. g. Öresund, by supposing that the sea-water outside in consequence 

 of its greater specific gravity displaced the fresher and lighter water in 

 the separated ocean-basin and thus flowed into it in the form of an 

 under-current. The same reasoning ought therefore to be applied to the 

 under-currents at the mouths of rivers. This explanation, however simple 

 it may appear, seems nevertheless on closer examination not to be satis- 

 factory. If we supposed the outflow from a river suddenly to stop, 

 as soon as the river had deposited a certain quantity of fresh water on 

 the surface of the sea, not only wotild assuredly a superficial current of 

 outflowing fresh water arise, but also an under-current of salt-water in 

 the opposite direction in consequence of the water-stratas differences of 

 pressure, and these would continue till the fresh water was carried off, 

 according to what has above been explained more fully in the case of 

 streams caused by heat and rainfall. But if the river continued to flow 

 with the same velocity, which caused the formation of the fresh-water- 

 stratum before the mouth, it seems clear, that water could not run away 

 in consequence of the excess of hydrostatic pressure more rapidly, than 



