THE SPECIFIC GRAVITY OF THE SEA, ETC. 215 



The season of observation, however, is southern fall and winter vice 

 northern summer. In January, Februar}^, and March, the waters 

 of the southern ocean are decidedly warmer, as at the opposite six 

 months they 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 study- 

 ing the laws of this thermal tide we are struck with the compensa- 

 tions 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 vforking out the geometry of 

 the 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 cnmiple 

 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 sm-ely 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 pertm'bations in the orbits 

 of planets ; but the physicist is no more at liberty to despise these 

 than the astronomer is to neglect those. 



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

 sea water of the south- presented bv the diaoframs of Plate X., is to put the 



era cooler and heavier, ■■- . ■> ,^ • i ^ ix '1 



parallel for parallel, scas lu scaios, the occau m a balance, and to weign 

 norlthem hemisphere, ill the specific gra\dty bottle, the waters of the 

 northern with the waters of the southern hemisphere. By Figr. 2 

 it would appear that both the water and the air of the south At- 

 lantic are decidedly both cooler and heavier, parallel for parallel. 



