THE SiVLTS OF THE SEA. 243 



water ocean, and to go on as it does from the seas as tliey are. 

 In those regions, as in the trade-wind regions, where evaporation 

 is in excess of precipitation (§ 545), the general level of this 

 supposed sea would be altered, and immediately as much water 

 as is carried off by evaporation would commence to flow in from 

 north and south towards the trade-wind or evaporating region to 

 restore the level. On the other hand, the winds would have 

 taken this vapour, borne it off to the extra-tropical regions, and 

 precipitated it, we will suppose, where precipitation is in excess 

 of evaporation. Here is another alteration of sea level by 

 elevation instead of by depression ; and hence we have the motive 

 power for a surface current from each pole towards the equator, 

 the object of which is only to supply the demand for evaporation 

 in the trade-wind regions — demand for evaporation being taken 

 here to mean the difference between evaporation and precipitation 

 for any part of the sea. Now imagine this sea of uniform tem- 

 perature to be suddenly stricken with the invisible wand of heat 

 and cold, bringing its waters to the various temperatures at which 

 they at this instant are standing. This change of temperature 

 would make a change of specific gravity in the waters, which 

 would destroy the equilibrium of the whole ocean ; upon this a 

 set of currents would immediately commence to flow, namely, a 

 current of cold and heavy water to the place of the warm, and a 

 current of warm and lighter to the place of the cold. The motive 

 230wer of the currents thus created would be difference of siDecifio 

 gravity arising from difference of temperature in fresh water. 

 We have now traced the effect of two agents, which, in a sea of 

 fresh water, would tend to create currents, and to beget a system 

 of aqueous circulation ; but a set of currents, and a system of 

 circulation which, it is readily perceived, would be quite feeble 

 in comparison with those which we find in the salt sea. One of 

 these agents would be employed in restoring, by means of one or 

 more polar currents, the water that is taken from one part of the 

 ocean by evaporation, and deposited in another by precipitation. 

 The other agent would be employed in restoring, by the forces 

 due to difference of specific gravit}^, the equilibrium, which has 

 been disturbed by heating, and of course expanding, the waters 

 of the .torrid zone on one hand, and by cooling, and consequently 

 contracting, those of the frigid zone on the other. This agency, 

 would, if it were not modified hy others, find expression in a 

 system of currents and counter currents, or rather in a set of 



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