TEMPERATURE OF VALLEYS. 127 



generally colder as we ascend, yet from local circumstances the 

 heat of the earth being increased, the air becomes sometimes 

 very mild even in elevated districts. Valleys, it is well known, are 

 warmer than level plains in the same latitude on account of the 

 reflection of the sun's heat from neighboring hills and mountains. 

 In Switzerland, instances occur of spots of verdure in the midst 

 of perpetual snow and glaciers. We give below a view of the 

 glacier of Grindelwald in the Canton of Berne. Here woods 

 and meadgws border close upon the immense fields of ice, which 



descending from the upper regions, cover an extent of about 1200 

 square miles of territory. The ice is seen presenting innumer- 

 able peaks in the gorge between the mountains. It is said that 

 there are plains in the Hhnalayah mountains 15,000 feet above 

 the level of the sea which produce fine pasturages. 



Air when put in motion produces what is called wind, and the 

 variable distribution of heat throughout the atmosphere is the 

 main cause of wind, or flow of the air, for thus the local density 

 is constantly affected, and the equilibrium of the mass disturbed. 

 Winds are exhibited in various forms, breezes, high winds, gales, 

 hurricanes and tornadoes. These varieties depend chiefly upon 

 their different velocities, a velocity of twelve miles an hour mak- 

 ing a strong breeze; sixty miles, a high wind; one hundred miles 

 a hurricane. 



The force of the wind when moving with the velocitvof a 

 hurricane or tornado, is almost incredible ; when we speak of the 

 geological changes that have passed over the face of our globe 



