On the general Causes of the Ocean-Currents. 9 



of the surface-stream, caused by the change of level, whereby the con- 

 traction in the vertical direction is diminished, and accordingly the difte- 

 rences of pressure, that cause the under-current, exaggerated. 



The effect then, that the earth's warming in the neighbourhood 

 of the equator and cooling in that of the poles would have upon the 

 position and motion of the strata in the ocean, if there ivere no other 

 modift/inij circumstances^ would be the following: The level of the sea 

 would be changed by rising at the equator and sinking at the poles. 

 The deviations from the normal level, which would thus arise, would 

 in some measiire approximate to those, which according to the laws of 

 equilibrium in communicating tubes, would be calculated to take place, 

 supposing the waters specihc weigiit at different depths to be accurately 

 known. Two superficial currents of warm water would flow from the 

 equator towards the, poles, and two less rapid under-currents of cold 

 water in the opposite direction. All these streams would diffuse them- 

 selves uniformely over the whole breadth of the ocean, with only such 

 modifications as would arise from the form of the continents and the ro- 

 tation of the earth. There would in short be a circulation «sustained by 

 difference of temperature alone», in its nature something similar to what 

 l):r Carpenter has described, though in its causes different. 



The great effect, that the cooling of the ocean's surface has upon 

 the temperature of the deeper waters, has been fully developed by D:r 

 Carpenter and I have therefore barely touched upon it. I do not how- 

 ever consider, that the account he has given of the oceanic circulation, 

 arising from variation of temperature *), agrees with what I have above 

 described. If I rightly understand him, he supposes the ocean-surface 

 at the poles to be on a level with that at the equator. In this case a 

 pillar of water at the poles, extending to a certain depth below the 

 surface, would exert a considerably greater pressure than a correspon- 

 ding (uie at the equator on account of the formers greater specific gra- 

 vity, and tliis difference of pressure ought, according to D:r Carpenter, 

 to be the driving force, that elevates tlie water at the equator. The 

 driving force producing the circulation lies, according to his view, not 



') W. B. Caepenter: On the Latest Scientific Researches in the Mediterranean, 

 Proceed, of K. Inst, of G. Br., 10 March 1S71; «Ocean circulation». Contemporaneous 

 Kewicw, Sept. 1875. 



Nova Acta Reg. Soc. Sc. Ups. Ser. III. 2 



