§ 444,445. THE SPECIFIC GRAVITY OF THE SEA, ETC. 221 



sea water (§ 40i), show that we have not yet data sufficient to 

 establish the depth, or even the existence of such an isothermal 

 floor (§ 440) all the way from pole to pole. 



444. ''The physical consequences of this great law, should it 

 Data for Plate X. be fouud Completely verified by farther research, 



are in the last degree important." The observations which fur- 

 nished the data for Fig. 1 were made in the North Pacific be- 

 tween 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. It is well to bear this difference as to season north and 

 AthermaitideMtebbs south lu mind, and to compare these curves with 

 and flows once a year. i\^q^q of thc thermal charts ; for the two together 



indicate the existence in the ocean of a thermal tide, which, as be- 

 fore stated (§ 439), ebbs and flows but once a year. By this figure 

 the South Atlantic appears to be cooler and heavier than the north- 

 ern. The season of observation, however, is southern fall and win- 

 ter vice northern summer. In January, February, and March, the 

 waters of the southern ocean are decidedly warmer, as at the oppo- 

 site six months they are decidedly cooler, parallel for parallel, than 

 those of the northern oceans. Thus periodically differing in tem- 

 perature, the surface waters of the two hemispheres vary also in 

 specific gravity, and give rise to an annual ebb and flow — an up- 

 per 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 adj astments that are allotted to it in the mech- 

 anism 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 ge- 

 ometry of the heavens. He finds that it is these small quantities 

 which make harmony in the celestial spaces ; and so, too, it is the 

 gentle forces like this in the waters which preserves the harmony 

 of the seas. Equatorial and polar seas are of an invariable temper- 

 ature, 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 



