'224: PHYSICAL GEOGKArilY OF TUB SEA, AND ITS METEOROLOGY. 



heavens. He finds tliat it is these small quantities whicli make 

 the music of the spheres; and so, too, it is the gentle forces like 

 this in the waters Avhicli preserve the harmony of the seas. 

 J^lquatorial 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 con- 

 traction 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 mis- 

 taken its rate and velocity. These movements are well repre- 

 sented on the thermal charts. The tiny ripplings of this feeble 

 tide have, we ma}^ 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 libert}^ to despise these than the 

 astronomer is to neglect those. 



446. Sea ivater of the southern cooler and heavier, parallel fcr 

 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 w^eigh in the specific-gravity bottle, the waters of 

 the northern with the w^aters of the southern hemisiDhere. 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 w^e a series of observations 

 the converse of this, viz., wdnter 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 tempera- 

 ture, and consequently the specific gravity of the w^aters of each 

 must change wdth the seasons. A diagram — had w^e the data for 

 such a one— to show these changes, w^ould be very instructive ; it 

 would show beautifully, by its marks, the ebb and flow of this new- 



