surface currents of warm and lighter water, from the equator towards the poles, and in another set of under currents of cooler, dense, and heavy water from the poles towards the equator.
469. Currents without wind.—Such, keeping out of view the influence of the winds, which we may suppose would be the same whether the sea were salt or fresh, would be the system of oceanic circulation were the sea all of fresh water. But fresh water, in cooling, begins to expand near the temperature of 40°,[1] and expands more and more till it reaches the freezing-point, and ceases to be fluid. This law of expansion by cooling would impart a peculiar feature to the system of oceanic circulation were the waters all fresh, which is not necessary here to notice farther than to say it cannot exist in seas of salt water, for salt water (§ 405) contracts as its temperature is lowered, and until it passes its freezing-point. Hence, in consequence of its salts, changes of temperature derive increased power to disturb the equilibrium of the ocean. If this train of reasoning be good, we may infer that, in a system of oceanic circulation, the dynamical force to be derived from difference of temperature, where the waters are all fresh, would be quite feeble; and that were the sea not salt, we should (§ 462) probably have no such current in it as the Gulf Stream. So far we have been reasoning hypothetically, to show what would be the chief agents, exclusive of the winds, in disturbing the equilibrium of the ocean were its waters fresh and not salt. And whatever disturbs equilibrium there may be regarded as the primum mobile in any system of marine currents.
470. Influence of salts and evaporation.—Let us now proceed another step in the process of explaining and illustrating the effect of the salts of the sea in the system of oceanic circulation. To this end, let us suppose the imaginary ocean of fresh water suddenly to become that which we have, namely, an ocean of salt water which contracts as its temperature is lowered (§ 441) till it reaches 25°.6. Let evaporation now commence in the trade-wind region, as it was supposed to do (§ 468) in the case of the fresh-water seas, and as it actually goes on in nature—and what takes place? Why, a lowering of the sea level, as before. But as the vapour of salt water is fresh, or nearly so, fresh water only is taken up from the ocean; that which remains behind is therefore more salt. Thus, while the level is lowered in the salt sea, the equilibrium is destroyed because of the saltness of the water; for
- ↑ 39° 5.
