heavens. He finds that it is these small quantities which make the music of the spheres; and so, too, it is the gentle forces like this in the waters which preserve the harmony of the seas. Equatorial and polar seas may be of an invariable temperature, but in middle latitudes the sunbeam has power to wrinkle and crumple the surface of the sea by alternate expansion and contraction of its waters. In these middle latitudes is the cradle of the tiny thermal tide here brought to light; feeble, indeed, and easily masked are its forces, but they surely exist. It may be that the thermometer and hydrometer are the only instruments which are nice enough to enable us to detect it. Its footprints, nevertheless, are well marked in our tables showing the thermal dilatation of sea water. The movements of the isothermal lines, marching up and down the ocean, show by signs not to be mistaken its rate and velocity. These movements are well represented on the thermal charts. The tiny ripplings of this feeble tide have, we may be sure, their office to perform in the general system of aqueous circulation in the sea. Their influence may be feeble, like small perturbations in the orbits of planets; but the physicist is no more at liberty to despise these than the astronomer is to neglect those.
446. Sea water of the southern cooler and heavier, parallel for parallel, than sea water of the northern hemisphere.—The problem that we now have in hand, and which is represented by the diagrams of Plate X., is to put the seas in scales, the ocean in a balance, and to weigh in the specific-gravity bottle, the waters of the northern with the waters of the southern hemisphere. By Fig. 2 it would appear that both the water and the air of the south Atlantic are decidedly both cooler and heavier, parallel for parallel, than the waters of the north Atlantic; but this difference may be more apparent than real; for the observations were made in the northern summer on this side, and in the southern fall and winter on the other side of the equator. Had we a series of observations the converse of this, viz., winter in the north Atlantic, summer in the south, perhaps the latter would then appear to be specifically the lighter; at any rate, the mean summer temperature of each Atlantic, north and south, is higher than its mean winter temperature, and consequently the specific gravity of the waters of each must change with the seasons. A diagram—had we the data for such a one—to show these changes, would be very instructive; it would show beautifully, by its marks, the ebb and flow of this new-
