On the general Causes of the Ocean-Currents. 3 



pressure from that latitude to the pole it ought at 75° N. Lat. to stand 

 about 11 centimètres and at 74" South Lat. about 40 centimètres higher 

 than at the tropics. If the difference in the atmospheric pressure were 

 absolutely constant, this difference of level could not give rise to any 

 stream, but the water-particles of the inclined surfaces would move with 

 equal facility in any direction. Only when, from some other cause, the 

 water streams from one latitude to another, can the difference of the 

 mean barometric pressure have a modifying influence on the motion of 

 the stream. The streams, that flow from the tropics towards the poles, 

 will have their movement very slightly facilitated, those flowing in the 

 opposite direction will be retarded, but accelerated again, as soon as 

 they have passed the tropics. 



Every variation of the atmospheric pressure must cause water to 

 flow from one place to another; but these changes, being only occasional 

 and of short continuance, can have only a trifling effect upon the real 

 oceanic currents, although they may have a more considerable temporary 

 influence on the streams in straits, which unite separate oceanic basins 

 with eachother. 



The changes of level, caused by heat and cold, evaporation and 

 rainfall, on the other hand may give rise to constant movements in the 

 sea-water; for these operate with an average constant difference of force 

 on different portions of the ocean.' I have already said, that they cause 

 streams in consequence of changes in the level of the sea. They how- 

 ever also alter the volume and specific gravity of the sea-water, the 

 latter partly by changing its saltness and partly its temperature. 



Illach of these changes has its effect upon the ocean's circulation, 

 and I cannot consider it as a matter of indifference, whether we assume 

 one or the other of them as the immediate cause of the resulting 

 stream. Maury, and many others after him, have pointed out as the 

 cause the variation of the specific gravit}^ of the sea-water. When for 

 example he in the chapter: «Tlie origin of Currents» epitomizes his view 

 of the subject, he uses the following words: ') «If we except the tides 

 and the partial currents of the sea, such as those which may be created 

 by the wind, we may lay do\vn as a rule, that all the currents of the 

 ocean owe their origin to difference of specific gravity between sea- 

 water at one place and sea-water at another; for wherever there is such 

 a difference, it is a difference that disturbs equilibrium and cur- 



') Mauky, Phys. Geography of the Sea. 1872, pag. 204. 



