RELATIONS OF AIR AND WATER TO TEMPERATURE AND LIFE. 271 



the suininer solstice and the plains in India and China become heated 

 by the torrid sun the wind changes and blows toward the northeast. 

 At the meeting of the winds the monsoon breaks, and the cyclones of 

 India and the typhoons of China follow. They are soon over, and then 

 the monsoon blows over Indian Ocean and China Sea. All India, Kash- 

 mir, and western Tibet, Farther India, Annam, and eastern China and 

 Japan are well watered, 50 inches of rain falling in a year in some parts 

 of India. 



In these countries there are generally six months of rainy season and 

 six months of dry. In parts of India the water of the rainy season is 

 stored in large reservoirs for irrigation in the dry season, while in China 

 numerous canals between the different rivers in like manner irrigate the 

 land. India and China are among the richest countries of the world 

 and have the densest population, though destined to be surpassed in 

 the future by the population of the Amazon and Mississippi valleys. 



We have thus seen the effects of the winds and ocean currents in 

 modifying the climate and in enriching the great valleys of South Amer 

 ica and North America, of Europe, India, China, and Japan. 



DESERTS OR BASINS. 



About one-fifth of the territory in each continent is arid and desert 

 land. With one or two possible exceptions these arid regions are basins, 

 where the rivers and rain fall either run into salt lakes or are lost in the 

 desert and never reach the ocean. These deserts are caused by the 

 winds which blow either from colder over warm areas and are there- 

 fore dry, or over vast plains or mountainous regions upon which they 

 have precipitated their moisture. 



The average rain fall on the great deserts does not exceed 10 inches a 

 year, and the evaporation is usually greater than the rain-fall. They 

 are situated generally between the twentieth and fortieth degrees of 

 north latitude and between the twentieth and thirtieth degrees of south 

 latitude. In the northern belt are the Carson and other basins of 

 Nevada, the Salt Lake of Utah, the desert of Sahara, Arabia, Persia, 

 the Aral-Caspian desert, the Tanin Gobi and Mongolia desert. In the 

 southern belt is the desert of Atacama in South America, Kalahari in 

 South Africa, and the Australian deserts. These basins in the north- 

 ern belt contained formerly lakes much greater than are now found in 

 either of the continents. 



Salt Lake was formerly much larger and deeper, for its waters once 

 beat upon shores 1,000 feet higher up the mountain sides than at i)res- 

 ent. Its waters then found their way to the ocean. This was probably 

 in the ice age, when the surrounding mountains were covered with snow 

 and great glaciers, and the evaporation was much less than the rain-fall 

 and the water from the melting glaciers. 



