486 



PHYSICAL GEOGRAPHY. 



south latitude is higher than at the equator, an evidence of a higher temperature. With 

 this exception, if admitted to be one, the decrement of heat proceeds gradually as we 

 travel along the same level from the line to the poles. 



2. The temperature of countries is largely affected by the extent of their elevation 

 above the level of the sea. It is well known that as we ascend in the atmosphere the 

 cold increases, an effect due to the rarefaction of the air, and to the circumstance of being 

 further from the heat reflected from the surface of the earth. "We must travel several 

 hundred miles along the surface from the equator, before we become sensible of a 

 diminished temperature; but an ascent there of only a thousand yards will sink the 

 thermometer 10, or 1 for 310 feet, and the decrease x>f heat will go on as we ascend 

 higher, with a little irregularity, till at the height of somewhat less than 16,000 feet, or 

 about 3 miles, we come to the line of perpetual congelation. The ratio of the diminu 

 tion of temperature usually given, is 1 for 300 feet of altitude ; 2 for 595 feet ; 3 for 

 872 feet ; 4 for 1124 feet ; 5 for 1347 feet ; and 6 for 1539 feet. In the temperate zone 

 generally, if one site is a thousand yards higher than another adjoining, it will have a 

 climate 12 colder; and the higher the latitude the lower the snow-line becomes,- till it 

 osculates with the surface of the earth in the frigid zone. The following diagram 



Surf act of theEcu-Lh. 



represents the line of perpetual snow, forming the arc of an ellipsoid, passing over the 



equator, from pole to pole. 



But this important boundary-line, as it exists in nature, exhibits no continuous curva 

 ture, though attaining a considerable elevation at the equator, and level with the surface 

 in high latitudes. It repeatedly oscillates, suddenly ascends and descends, under control 

 of the physical peculiarities of different regions. The snow-line of the Andes, at the 

 equator, is found at the height of 15,748 feet ; but not a particle of permanent snow was 

 seen by Mr Pentland at the following sites on the Bolivian part of the chain, though in 

 from 17 to 19 of south latitude I 

 Latitude. Elevation. 



17 18' . . 15,610 



16 42' . . 16,250 



19 45' . . 15,913 



19 36' . . 16,370 



Pass of Cliulhuuquaui, ..... 

 Mountain of La Golofa, ..... 



Mountain of Porco, ..... 



Cerro di Potasi, summit of the celebrated metalliferous mouuLain, 



A remarkably great and sudden fall of the Andean snow-line occurs further south, unpar 

 alleled in any other part of the globe. 



Thus, within the distance of only 10 of latitude, the snow line experiences a difference 

 of elevation of 9000 feet, an effect attributed to the fact of Chiloe being covered with forest 

 trees dripping with moisture, indicating a clouded sky and little heat in summer, while in 

 Central Chili rain does not fall for the seven summer months ; the sky is generally clear, 

 and the climate hotter. The subjoined table gives the height of the curve of congelation 

 in different latitudes, exclusively from calculation founded upon the known law of the 

 decrease of heat by elevation. 



