Oft ITS EXTERIOR. THERMAL SPRINGS. 195 



thirds of the whole surface of the earth (according 

 to the present state of geographical discovery in the 

 polar regions of either hemisphere, the ratio of sea and 

 land may be taken as 8:3) are sea-bottom. This is in 

 immediate contact with strata of water, slightly salt, 

 and which, following the order of superposition corre- 

 sponding to their temperature of maximum density 

 (39 0< 1 Fahr.), are of an icy coldness. Exact observations 

 by Lenz and du Petit-Thouars have shown that in the 

 midst of the tropics, where the surface of the ocean has 

 a temperature of about 80, water of 36^- Fahr. may 

 be drawn up from depths of seven or eight hundred 

 fathoms ; a phenomenon which manifests the existence 

 of submarine currents from the polar regions. The 

 consequences of this sub-oceanic constant cooling pro- 

 cess of by far the larger portion of the earth's crust, 

 deserve a greater measure of attention than they have 

 hitherto received. Rocks and islands of small circum- 

 ference rising like cones from the bottom of the sea 

 above the surface of the waters, and narrow isthmuses, 

 such as Panama and Darien, washed by wide oceans, 

 must present a different internal distribution of heat, to 

 that existing in masses of similar form and density in 

 the interior of continents. In a lofty mountain-island, 

 the submarine portion of its vertical height is in contact 

 with a fluid of which the temperature increases from 

 below upwards ; while, as the rocky strata emerge from 

 the sea, they come in contact, under the influence both 

 of the sun's direct rays and of free outward radiation, 

 with a gaseous fluid in which the temperature decreases 

 with increasing height. Similar thermal relations, of 

 opposite decrease and increase in the vertical line. 



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