THE SPECIFIC GRAVITY OF THE SEA, ETC. 223 



443. Sea wcder at summer more expansible than sea water at luinter 

 temperature. — All these experiments unite in showing that sea 

 water at equatorial temperatures is many times more expansible 

 than sea water at polar temperatures ; that is, sea water, ac- 

 cording to its rate of dilatation (§ 441), will expand about seven- 

 teen times as much for 5", when its temperature is raised from 

 85°, as it will when raised from 28^ ; and yet, according to Plate 

 X., the curves oftemperature and specific gravity are sj^mmetrical 

 in.polar, non-symmetrical in equatorial seas. These. experiments, 

 and the compressibility of sea water (§ 404), show that we have 

 not yet data sufficient to establish the depth, or even the ex- 

 istence of such an isothermal floor all the w^ay from pole to pole. 



444. Data for Plate X. — *' The phj^sical consequences of this 

 great law, should it be found completely verified by farther re- 

 search, are in the last degree important." The observations 

 which furnished the data for Fig. 1 were made in the North 

 Pacific between the months of August, 1855, and April, 1856, and 

 in the South Pacific dm-ing April and May ; whereas for Fig. 2 

 the southern observations were made in May and June, the 

 northern in June and July. 



445. A thermal tide : it ebbs and floivs once a year.— It is well to 

 bear this difference as to season north and south in mind, and to 

 compare these curves with those of the thermal charts ; for the 

 two together indicate the existence in the ocean of the thermal 

 tide, which, as before stated, ebbs and flows but once a year. 

 By this. figure the South Atlantic appears to be cooler and 

 heavier than the northern. The season of observation, however, 

 is southern fall and winter vice northern summer. In January, 

 Febniary, and March, the waters of the southern ocean are de- 

 cidedly warmer, as at the opposite six months the}' 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 studying the laws of this 

 thermal tide we are struck with the compensations and adjust- 

 ments that are allotted to it in the mechanism of the sea ; for 

 these feeble forces in the w^atcr remind one of the quantities of 

 small value — residuals of compensation — with which the astro- 

 nomer has to deal when he is working out the geometry of the 



