EVAPORATION AND RAIN. 19 



of rain within the tropics, except in desert 

 places, is about 100 inches, or three times 

 greater than in the middle latitudes ; and about 

 nine times greater than in the polar regions. 

 Hence the enormous size of the tropical rivers, 

 compared with those of the higher latitudes. For 

 example, it has been found that the Orinoco, which 

 drains only 400,000 square miles of territory, dis- 

 charges more water into the sea than the Missis- 

 sippi, which drains 1,350,000 square miles.* It 

 will also be shewn, that the quantity of light- 

 ning throughout the earth, diminishes from the 

 torrid zone ; where during the rainy season, it 

 occurs almost every day, for nearly half the 



* It has been estimated by geographers, that about thirty-three 

 cubic miles of water are discharged daily into the sea by all the 

 rivers of the earth. So that if this calculation be correct, or even 

 an approximation to the truth ; and if the same proportion of 

 rain falls into the sea as on a given area of dry land, it follows 

 that nearly 140 cubic miles of water must be carried into the 

 atmosphere every twenty-four hours, and again precipitated in 

 the form of rain and snow. Such are the vast forces which caloric 

 is continually exerting in the phenomena of meteorology, without 

 any reference to those violent commotions of the atmosphere 

 termed hurricanes, tornadoes, and other less impetuous winds. 

 Nor is it unworthy of passing notice ; that as the quantity of 

 water diffused through the atmosphere is in proportion to the 

 heating power of the sun, the barometer stands higher, on an 

 average, within the tropics than in the middle or polar latitudes ; 

 as proved by the observations of Erman, laid before the French 

 Academy of Sciences, after my chapter on the barometer had been 

 printed. More water is also converted into steam during the heat 

 of the day, and more steam is condensed into water during the 

 coldness of night, which is the most convenient period for rain, 

 as it does not then impede the labours of husbandry. 



