443. Sea water at summer more expansible than sea water at winter temperature.—All these experiments unite in showing that sea water at equatorial temperatures is many times more expansible than sea water at polar temperatures; that is, sea water, according to its rate of dilatation (§ 441), will expand about seventeen times as much for 5°, when its temperature is raised from 85°, as it will when raised from 28°; and yet, according to Plate X., the curves of temperature and specific gravity are symmetrical in.polar, non-symmetrical in equatorial seas. These. experiments, and the compressibility of sea water (§ 404), show that we have not yet data sufficient to establish the depth, or even the existence of such an isothermal floor all the way from pole to pole.
444. Data for Plate X.—"The physical consequences of this great law, should it be found completely verified by farther research, are in the last degree important." The observations which furnished the data for Fig. 1 were made in the North Pacific between the months of August, 1855, and April, 1856, and in the South Pacific during April and May; whereas for Fig. 2 the southern observations were made in May and June, the northern in June and July.
445. A thermal tide: it ebbs and flows once a year.—It is well to bear this difference as to season north and south in mind, and to compare these curves with those of the thermal charts; for the two together indicate the existence in the ocean of the thermal tide, which, as before stated, ebbs and flows but once a year. By this. figure the South Atlantic appears to be cooler and heavier than the northern. The season of observation, however, is southern fall and winter vice northern summer. In January, February, and March, the waters of the southern ocean are decidedly warmer, as at the opposite six months the}' are decidedly cooler, parallel for parallel, than those of the northern oceans. Thus periodically differing in temperature, the surface waters of the two hemispheres vary also in specific gravity, and give rise to an annual ebb and flow—an upper and an under tide—not from one hemisphere to the other, but between each pole and equator. In contemplating the existence and studying the laws of this thermal tide we are struck with the compensations and adjustments that are allotted to it in the mechanism of the sea; for these feeble forces in the water remind one of the quantities of small value—residuals of compensation—with which the astronomer has to deal when he is working out the geometry of the
