Page:Physical Geography of the Sea and its Meteorology.djvu/269

water ocean, and to go on as it does from the seas as they 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 temperature 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 power of the currents thus created would be difference of specific 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 gravity, 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 by others, find expression in a system of currents and counter currents, or rather in a set of