On the GENER4L CaU.SES OF THE OcEAN-CuRRENTS. 47 



strength of the Gulf-stream. Even if the access of river-water and the 

 greater amount of rain in the Gulf of Mexico are not sufficient to pro- 

 duce an absolute rise of the water's level, they must at any rate operate 

 essentially to prevent a sinking of the surface through evaporation, and 

 thus effect a rise relatively to the ocean outside the Gulf. To this must 

 be added also the influence of the reaction-streams of rivers, if indeed 

 these, as is probable, are fed from the ocean itself. We should remem- 

 ber that river-water, which is poured into an ocean-gulf, even if it had 

 just time to evaporate before leaving the gulf, must nevertheless pro- 

 duce an out-flowing stream, because the reaction-currents, which the 

 river-water has drawn into the gulf, must somewhere or other find their 

 way out again. 



But one of the chief causes of the strength of the Atlantic Gulf- 

 stream and of the great extent towards the North-east of the surface- 

 drift, which forms a continuation of its motion, I conceive to be the 

 free communication between the Atlantic and the Arctic-Oceans '), and 

 the relation that thence arises between the two surface-streams, namely 

 the ascending warm and salt, and the descending cold and diluted. We 

 remember that this latter stream was a secondary effect of rainfall, rain- 

 forced by the surface-stream, which cold produced by forming ice, which 

 is afterwards dissolved by the water. It may perhaps seem as if this 

 descending stream ought, if the above given grounds for the explanation 

 of the ocean-streams be accepted, to oppose an insuperable obstacle to 

 the progress of the ascending stream, in as much namely as the chan- 

 ges of level, that give rise to the streams, appear to operate at every 

 instant to establish a higher sea-level in higher than in lower latitudes. 

 But we must recollect, that the causes most effective towards this end, 

 viz: the absolute amount of rainfall and evaporation, produced a very 

 deep stream and thus in proportion to the greatness of the change of 

 level, a relatively small difference of level. This difl'erence might fur- 

 ther, as I have above shown, be compensated by the eftect of heat, 

 without the surface-current, which was produced by heat, being debilitated. 



The difference of level again, which causes the descending surface- 

 current, depends exclusively on the relation between the surface-stratas 

 specific iceights^ and must therefore be very small, and does not, as far 



') Dr. Carpentei! has experieuced the same opinion though upon different 

 gi'onnds, namely on the ground of a supposed suction, occasioned by the water sin- 

 king in the cold polar regions of the ocean. 



