OCEANIC CIRCULATION. 251 



surface,' by trade-wind action would be a surface cur- 

 rent. The two cases are wholly dissimilar, I must, 

 however, admit that my case is one of extreme diffi- 

 culty regarded as a problem in hydrodynamics. It is 

 so difficult that I do not believe it can be solved even 

 after the very imperfect fashion in which hydrodyna- 

 mical problems have hitherto perforce been dealt with. 

 When the physics of hydrodynamics have been treated 

 by mathematicians like the physics of astronomy, or 

 rather when they can be so treated, it may be possible 

 to deal with this problem. Unless I greatly mistake, 

 however, in such a then, we shall find a never. 



I do not see how the action of the cause I have 

 considered is affected by the circumstance that the 

 equatorial heat does not show any effects below 200 

 fathoms ; for the cause is in its very nature a surface 

 one. But I would remark that so far as continuity of 

 action is concerned, the equatorial heat seems at least 

 on a par with the polar cold. For as the aqueous vapour 

 rises it finds its way to regions where the atmospheric 

 circulation is at work to carry it away (it is only the 

 surplus quantity which is condensed into clouds, and 

 even these are in great part carried away) ; and thus 

 the process of evaporation can hardly be exhausted. 

 Even at night, though in a modified manner, the eva- 

 poration must continue. But the action of the polar 

 cold, though it is continuous in the sense that the 

 increase of cold extends to great depths, yet has this 

 great difficulty to contend with, that the descending 

 water must perforce wait until room is made for it by 



