TERRESTRIAL HEAT. 



very rarely higher than that of the water, and in the polar 

 regions the air is never found as -warm as the surface of the 

 water. It is, on the contrary, in general at a very much lower 

 temperature. 



38. Much uncertainty prevails as to the thermal phenomena 

 manifested in the vast collections of water which cover the greater 

 part of the surface of the globe. It appears, however, to be 

 admitted that the currents caused by the difference of the pressures 

 of strata at the same level in the polar and equatorial seas, produce 

 an interchange of waters, which contributes in a great degree to 

 moderate the extreme thermal effects of these regions, the current 

 from the pole reducing the temperature of the equatorial waters, 

 and that from the line raising the temperature of the polar waters 

 and contributing to the fusion of the ice. A superficial current 

 directed from the line towards the poles carries to the colder 

 regions the heated waters of the tropics, while a counter current 

 in the inferior strata carries from the poles towards the line the 

 colder waters. Although the prevalence of these currents may be 

 regarded as established, they are nevertheless modified, both in 

 their intensity and direction, by a multitude of causes connected 

 with the depth and form of the bottom, and the local influence of 

 winds and tides. , 



39. The stupendous mass of water in the solid state which forms 

 an eternal crust encasing the regions of the globe immediately 

 around the poles, presents one of the grandest and most imposing 

 classes of natural phenomena. The observations and researches of 

 Captain Scoresby have supplied a great mass of valuable informa- 

 tion in this department of physical geography. 



40. Upon the coasts of Spitzbergen and Greenland vast fields of 

 ice are found, the extent of which amounts to not less than twelve 

 to fifteen hundred square miles, the thickness varying from twenty 

 to twenty-five feet. The surface is sometimes so even that a 

 sledge can run without difficulty for an hundred miles in the 

 same direction. It is, however, in some places, on the contrary, 

 as uneven as the surface of land, the masses of ice collecting in 

 columns and eminences of a variety of forms, rising to heights of 

 from twenty to thirty feet, and presenting the most striking and 

 picturesque appearances. These prodigious crystals sometimes 

 exhibit gorgeous tints of greenish blue, resembling certain varieties 

 of topaz, and sometimes this is varied by a thick covering of snow 

 upon their summits, which gives them the appearance of cliffs of 

 chalk or white marble, marked by an endless variety of form and 

 outline. 



41. These vast ice-fields are sometimes suddenly broken, by the 

 pressure of the subjacent waters, into fragments presenting a 



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