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 3 10 feet, and the decrease of 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 1 124 feet ; 5 for 1347 feet ; and 6 9 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 



Surface of the (trth 10 20 30 40 SO 6'0 70 SO' 



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

 equator, from pole to pole. 



Making allowance for the fact before referred to, that the snow-line of the Andes is 

 higher at some distance from the equator 1000 feet, according to a number of observa- 

 tions by Mr. Pentland between 14 and 17 south latitude the diagram should not 

 exhibit a continuous curvature, but be corrected as follows : 



S it rface of in e Earth 



80 



The diagram only conveys an approximate idea of the course of the line of perpetual 

 snow from the poles to the equator, for it repeatedly oscillates, and suddenly descends and 

 ascends, under control of the physical peculiarities of different regions. One of its 

 most remarkable flexures, unparalleled in any other part of the world, occurs on the west 

 coast of South America. 



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. 



