42 F. L. Ekmam, 



difference will, as we find, be largely increased. What then ought to 

 be the final result of these changes of level? Will it be a single current 

 towards the equator, driven by the difference of level arising from eva- 

 poration and rainfall, diminished by that arising from heat? Or will it 

 be two opposite streams, the one driven by the former difference undi- 

 minished, and the other by the latter difference? The answer to this 

 question is evident from what has proceeded. The difference of level, 

 produced by the combined effects of evaporation and rainfall, disturbed 

 the equilibrium of the ocean down to the bottom, that arising from heat 

 disturbed it only to a certain limited depth. The result will accordingly 

 be two streams, one of large section, towards the equator, the other a 

 shallow surface-current of warm water, from the equator; and the former 

 stream, as being caused by a more rapid change of level, will cai'ry to- 

 wards the equator far more water in a unit of time, than the latter will 

 carry from it. In addition to these two motions there will be a third, 

 viz a surface-stream, descending towards lower latitudes, caused hj the 

 disturbance of the superficial strata's equilibrium, which was the second 

 consequence of rainfall. 



The origin of these vast streams then is to be looked for in di- 

 sturbances of the ocean's level so small, that their daily amount can 

 only be expressed in millimètres and fractions of millimètres. But, when 

 these little changes of level begin to work upon the quiescent mass, the 

 difference of level increases in a more rapid ratio than the stream it 

 produces, and, when both at last have reached a constant state, the dif- 

 ference of level may easily be considerable. 



From the fact, that the change of level produced by evaporation 

 and rainfall is greater than that resulting from heat, we are not forced 

 to conclude, that the sea-level around the polarcircle is higher than 

 at the equator: for the descending current, which the former in combi- 

 nation produce, has a far greater depth than the ascending warm stream, 

 and the difference of level, which occasions the former stream, may 

 therefore much sooner arrive at its maximum than that, which has to 

 drive the latter. As regards this question I may refer the reader to 

 what I have above more fully said, regarding the motion of surface- 

 currents in comparison with under-streams. 



In order to get a clearer view of the nature of the streams be- 

 fore us, let us first endeavour to understand how they might originate, 

 if the ocean were previously at rest. Here we must recollect, that, 

 since the maximum density of sea-water takes place at — 4° Centig. 



