Page:Physical Geography of the Sea and its Meteorology.djvu/230

204 out heat, raising the temperature, and changing the specific gravity of like quantities in the various thermal strata through which it has to pass. Thus heat is conveyed from the top to the bottom of the sea, there to be liberated and impart to its waters dynamical force for their upward movement. This is the power we paused to search for: whatever be its amount it is in the nature of a vera causa, and we must therefore recognize it, if not as the sole agent, nevertheless as one of the principal agents which nature employs in the system of vertical circulation that has been ordained for the waters of the sea.

405. Assisted by its salts.—Now, but for the salts of the sea this process could not go on so long as the laws of thermal dilatation remain as they are for sea water. Unlike fresh water, which expands as it is cooled below 39°. 5, sea water contracts until it has passed its freezing-point and attained the temperature of 25°.6. Were it not for its salts, sea water once near the surface within the tropics would, by reason of its warmth and thermal dilatation, remain near the surface. Vertical circulation would be confined to polar seas, and many of the living creatures that inhabit its waters would perish for the lack of currents to convey them their food.

406. The origin of currents.—If we except the tides, and the partial currents of the sea, such as those that may be created by the wind, we may lay it down as a rule (§ 103) that all the currents of the ocean owe their origin to difference of specific gravity between sea water at one place and sea water at another; for wherever there is such a difference, whether it be owing to difference of temperature or to difference of saltness, etc., it is a difference that disturbs equilibrium, and currents are the consequences. The heavier water goes towards the lighter, and the lighter whence the heavier comes; for two fluids differing in specific gravity (§ 106), and standing at the same level, can no more balance each other than unequal weights in opposite scales of a true balance. It is immaterial, as before stated, whether this difference of specific gravity be caused by temperature, by the matter held in solution, or by any other thing; the effect is the same, namely, a current. That the sea, in all parts, holds in solution the same kind of solid matter; that its waters in this place, where it never rains, are not Salter than the strongest brine; and that in another place, where the rain is