422 THE FARMER AT HOME. 



that vapor contains a great quantity. This condensation is often 

 formed by the attraction of an electrical cloud, and hence arises that 

 sultry heat which, in summer, is often felt before rain, arid particularly 

 before a thunder storm. 



As the earth is the source of heat, distance from the earth must, 

 consequently, be a cause of cold ; and, in confirmation of this theory, 

 it is invariably found that cold increases in proportion to our elevation 

 in the atmosphere. Hence we find, even under the equator, moun- 

 tains of a certain height have their tops covered with snow. An 

 elevation of five hundred yards produces the same eflect as a distance 

 of five thousand miles from the equator. Accordingly, at an eleva- 

 tion of thirteen thousand feet we find the frosts of the frozen zone ; 

 and at fifteen and sixteen thousand feet, the mountains, based upon 

 the most scorching plains, are capped with perpetual snow arid ice. 



The heat of the atmosphere is further augmented by the accumu- 

 lation of the sun's rays at the surface of the earth. The rays are then 

 reflected into the air and to surrounding objects ; so that the reflected 

 heat is often greater than the direct heat of the sun. On this account, 

 the heat in valleys, where the heat is reflected by hills and mountains, 

 is sometimes very great. In an elevated valley in Switzerland, the 

 heat is so much increased by reflection, that in the centre there is a 

 spot of perpetual verdure, in the midst of perpetual snow and glaciers ; 

 and there are plains on the Himmaleh mountains, fifteen thousand 

 feet above the level of the sea, which produce fine pasturage ; and at 

 the height of eleven thousand feet, which is above the region of per- 

 petual snows on the Andes, in the same latitude, barley and wheat are 

 known to flourish. 



Countries that are uncultivated aud covered w r ith wood, are much 

 colder than those which are open and cultivated ; as the former pre- 

 vent the access of the solar rays to the earth, or to the snow which 

 they may conceal, and also prevent a greater number of evaporating 

 surfaces than the latter. To be convinced that the air of woody coun- 

 tries is rendered colder by the evaporation from the trees and shrubs, 

 it is only necessary to observe that a thick shade of trees is cooler than 

 the shelter of buildings. As the land is capable of receiving and re- 

 taining much more heat or cold, than water can imbibe, the vicinity 

 of the sea is also a circumstance which considerably aflects the tem- 

 perature of the air. The sea therefore moderates the heat in warm 

 climates, and the cold in higher latitudes. When the rays of the sun 

 strike upon the water, they will penetrate six or seven hundred feet, 

 if there be that depth ; and the heat will be diffused through the 

 whole mass, remaining till carried off by evaporation. Consequently, 

 in hot climates, the body of the ocean is much cooler than the land ; 

 and in cold ones it is warmer. 



TEMPERATURE OF THE EARTH. The circumstance of the 

 earth being flattened at the poles and protuberant at the equator, is 



