GEOUND ICE. 215 



considered, the under limit of permanent snow on the 

 hills appears to be very elevated. The true laws of the 

 altitude of this line have not yet been ascertained. 

 Correct measurements have shown that within the tropics 

 the snow line varies several thousand feet, even in the 

 same range of mountains, according to its different 

 aspects. The active radiation of the sun in the continuous 

 day of an arctic summer, in conjunction with the com- 

 paratively small winter deposit, must tend to elevate the 

 snow line ; while within the tropics, the effect of a vertical 

 sun is compensated by nocturnal terrestrial radiation, 

 and the deposition that attends the sudden cooling of an 

 atmosphere charged with moisture. The same or similar 

 causes must tend to vary the breadth of the mountain 

 zone, comprehended between the summer and winter snow 

 line. The east coast climate everywhere north of Canada 

 has an analogy to this zone, and not to its upper limit or 

 the permanent snow line. 



Another phenomenon intimately connected with the 

 mean temperature of a district, is the " ground ice " or 

 " permanently frozen subsoil." The lateral extent of this 

 substratum, its southern limits, and its thickness, are in- 

 teresting subjects of inquiry. It is well known that the 

 warmth excited by the sun's rays is conducted slowly and 

 progressively into the earth, the effects of seasons and 

 years following like tides, but becoming less sensible and 

 less distinct until they are blended, and at a certain depth 

 vanish altogether. Professor Forbes says, that " the 

 decrease of the annual range is common to the strata 

 of the air above the surface of the earth and to those 

 of the soil beneath ; both ultimately, no doubt, exhibit a 

 limit, first where the diurnal variations disappear, then 

 the annual." The cause, however, he states is different in 

 the two cases ; the one being chiefly the result of the 



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