222 THE PHYSICAL GEOGRArHY OF THE SEA. 



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, never- 

 theless, are well marked in our tables showing (§ 441) 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 represent- 

 ed on the thermal charts. The tiny ripplings of this feeble tide 

 have, we may be sure, their ofiice to perform in the general system 

 of aqueous circulation in the sea. Their influence may be fee- 

 ble, like small perturbations in the orbits of planets ; but the phys- 

 icist is no more at liberty to despise the one than the astronomer 

 is to neglect the other. 



446. The problem that we now have in hand, and which is rep- 

 sea water of the south- rescnted by the diagrams of Plate X., is to put the 



ern cooler and heavier, . , , . - , i i • i 



parallel for parallel, scas lu scalcs, thc occau lu a balaucc, ancL to weigh 



than sea water of the . , . ^ • ^ i j ^i . i j. • nil 



northern hemisphere, m the speciiic gravity Dottlc tuc watcrs 01 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 At- 

 lantic are decidedly cooler and likewise heavier, parallel for paral- 

 lel, 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 temper- 

 ature, 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-born tide of the ocean. By Fig. 1 the south Pacific also out- 

 weighs the north in specific gravity ; but here again the true dif- 

 ference, whatever it be, is somewhat masked by the time of year 

 when the observations were made. Those north were made dur- 

 ing the fall, winter, and spring ; those south, during the fall and 

 first winter months of that hemisphere. Nevertheless, the weight 

 of the observations presented on Plate X. does, as far as they 



