220 



THE PHYSICAL GEOGRAPHY OF THE SEA. 



in diameter, with specimens of average sea water obtained in mid- 

 ocean and near the equator. On the 12th of February, 1858, the 

 thermometer in the shade being 23°, I exposed this jar of water, 

 with a standard thermometer immersed, to the out-door tempera- 

 ture. When the thermometer in the jar reached 27°, small crys- 

 tals of ice, like macles of snow, were observed to form near the 

 bottom, to rise, and to increase as they rose. In truth, the phe- 

 nomenon presented most beautifully in miniature a snow-storm 

 reversed, for the flakes appeared literally to " fall upward ;" and 

 while it was "snowing up" in the jar, covering the top with ice, 

 the water in it rose in temperature from 27°.2 to 28°, thus 

 showing the maximum density of the water to be not above 27°.2. 

 As soon, and invariably as soon, as the first crystals of ice began 

 to appear, the water immediately rose to 28°, and there remained 

 as long as the process of congelation was going on. In some in- 

 stances the water was brought down, as in a confined vessel, to 

 18° before freezing ; but as soon as freezing commenced, the ther- 

 mometer would mount up to 28°. The same water was used for 

 the following series of observations upon the thermal changes of 

 the specific gravity of sea water, fresh water being the unit : 



Temperature 27°. 1 



" 28°. 3 



*' 28°. 8 



" 29°. 



*' 29°. 5 



" 30°.0 



" 32°.0 



*' 34°. 



• *' 34°. 4 



" 35°. 2 



Spec. grav. 1.0290 



" 89 



" 91 



" 885 



" 906 



" 885 



" 88 



" 88 



" 89 



" 89 



Temperature 38°.0 Spec 



" 43°.5 



'' 54°.7 



" 55°.5 



" 62°. 5 



" 63°.5 



" 64°.5 



" 80°.5 



" 88°. 3 



" *93°.3 



grav. 1.0287 



86 



775 



77 



69 



675 



665 



43 



30 



1.0221 



448. All these experiments unite in showing that sea water at 

 Fea water at eum- equatorial tcmpcratures is many times more ex- 

 Lie than sea water pausiblc than sca watcr at polar temperatures ; that 



at winter tempera- . ,. . r»Tij.j.* 



ture. IS, sea water, according to its rate oi ailatation 



(§ 441), will expand about seventeen 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 of temperature 

 and specific gravity are symmetrical in polar, non-symmetrical in 

 equatorial seas. These experiments, and the compressibility of 



* Specific gravity at 200° =0.9908. 



