CLOUDS, RAIN, SNOW, AND HAIL. 279 



and seventy-eight inches is precipitated. The explanation of this is to be 

 founa in the pecuhar location of the city, at the foot of lofty mountains, whose 

 summits are covered with perpetual snow ; against these the hot, humid air 

 from the sea is driven by the winds, condensed, and its excess of moisture 

 precipitated as rain. 



614. Some countries are entirely destitute of rain; in a part of Egypt it 

 never rains, and in Peru it rains once, perhaps, in a man's lifetime. Upon 

 the table-land of Mexico, in parts of Guatemala and California, rain is very 

 rare. But the most extensive rainless districts are those occupied by the 

 great desert of Africa, and its continuation eastward over portions of Arabia 

 and Persia to the interior of Central Asia, over the great desert of Gobi, the 

 table-land of Thibet, and part of Mongolia. These regions embrace an area 

 of five or six millions of square miles tliat never experience a shower. 



The cause of this scarcity is to be sought for in the peculiar conformation 

 of the country. 



In Peru, for example, parallel to the coast, and at a short distance from the 

 sea, is the lofty range of the Andes, the peaks of which are covered with 

 perpetual snow and ice. The prevailing wind is an east wind, sweeping from 

 the Atlantic to the Pacific across tlie continent of South America. As it ap- 

 proaches the west coast, it encounters this range of mountains, and becomes 

 eo cooled by them that it is forced to precipitate its moisture, and passes on 

 to the coast almost devoid of moisture. In Egypt and other desert countries, 

 the dry sandy plains heat the atmosphere to such an extent that it absorbs 

 moisture, and precipitates none. 



On the other hand, there are some countries in which it may be said to 

 always rain. In some portions of Guiana, in South America, it rains for a 

 great portion of the year. The fierce heat of the tropical sun fills the atmos- 

 phere with vapor, which returns to the earth again in constant showers as 

 the cool winds of the ocean flow in and condense it. 



615. The whole quantity of water annually precipitated as 

 ■whole esti- rain over the earth's surface is calculated to exceed seven 



mated yearly hundred and sixtv millions of tons. This entire amount ia 



quantity of 



rain? raised into the atmosphere solely by evaporation. It has been 



also calculated, that the daily amount of water raised by 

 evaporation from the sea alone, amounts to no less than one hundred and 

 sixty-four cubic miles, or about sixty thousand cubic miles annually. ^ 



During the months of October and November, the daily amount of evapo- 

 ration from the surface of the ocean, between the Cape of Good Hope and 

 Calcutta, is known to average three quarters of an inch from the wholo 

 surface. 



__ . . The amount of moisture constantlv present in the atmos- 



Intiucnces .ire phore of any country, exercises an important influence upon 

 fhe'Tnor^ture^rf ^hc physical system of the inhabitants, and upon their arts 

 the atmosphere f and professions. Tlie atmosphere of the northern United 

 States is uncommonly dry, much more so than in England or 

 Germany. To this in a great measure is owing the difference in the physical 



