344 Dr George Buist on the 



grees warmer than ice, it attacks the bases and saps the 

 foundations of the icebergs — themselves gigantic glaciers 

 which have fallen from the mountains into the sea, or which 

 have grown to their present size in the shelter of bays and 

 estuaries, and by accumulations from above. Once forced 

 from their anchorage, the first storm that arises drifts them to 

 sea, where the beautiful law which renders ice lighter than 

 the warmest water enables it to swim, and floats southward 

 a vast magazine of cold to cool the tepid fluid which bears it 

 along, — the evaporation at the equator causing a deficit, the 

 melting and accumulation of the ice in the frigid zone 

 giving rise to an excess of accumulation, which tends, 

 along with the action of the air, and other causes, to institute 

 and maintain the transporting current. These stupendous 

 masses, which have been seen at sea in the form of church 

 spires and Gothic towers and minarets, rising to the height of 

 from 300 to 600 feet, and extending over an area of not less 

 than six square miles, the mass above water being only one- 

 tenth of the whole, are often to be found far within the tropics. 

 A striking fact dependent on this general law, has just been 

 brought to light ; there is a line extending from pole to pole 

 at or under the surface of the ocean, where an invariable 

 temperature of 39 0, 5 is maintained. The depth of this varies 

 with the latitude ; at the equator it is 7200 feet — at lat. 56° it 

 ascends to the surface, the temperature of the sea being here 

 uniform throughout. North and south of this the cold water is 

 uppermost, and at lat. 70° the line of uniform temperature de- 

 scends to 4500. But these, though amongst the most regular 

 and magnificent, are but a small number of the contrivances 

 by which the vast and beneficent ends of Nature are brought 

 about. Ascent from the surface of the earth produces the 

 same change in point of climate as an approach to the poles ; 

 even under the torrid zone, mountains reach the line of 

 perpetual congelation at nearly a third less altitude than 

 the extreme elevation which they sometimes attain : at the 

 poles, snow is perpetual at the ground, and at the different 

 intervening latitudes reaches some intermediate point of con- 

 gelation betwixt 1000 and 20,000 feet. In America, from the 

 line south to the tropics, as also in Africa, within similar lati- 

 tudes, vast ridges of mountains covered with perpetual snow, 



