294 EVAPORATION AND CONDENSATION. 



points of the compass, or with mountains and 

 high lands, when thunder gusts follow. 



When I come to treat of winds it will be 

 shown, that the most extensive falls of rain in 

 the middle latitudes are produced by the meet- 

 ing of immense masses of air from opposite 

 quarters of different temperatures, as during 

 the equinoctial floods and storms. When both 

 contain as much vapour as they can support at 

 their respective temperatures, the amount of pre- 

 cipitation is of course the greatest. 



Corresponding with the experiments of Dalton, 

 is the well established fact, that the greatest 

 quantity of rain falls within the tropics, where 

 the average temperature is from 80 to 83 F, and 

 diminishes on to the regions of lowest mean 

 temperature, where it is about 0. Hence it is 

 that the largest rivers in the world are found in 

 the tropical regions, as the Amazon, La Plata, 

 and Orinoko of S. America ; and the Indus, 

 the Ganges, the Nile, the Tigris, &c. of the 

 eastern continent. The summer temperature of 

 the Mississippi Valley is also tropical, and it is 

 well known that much more rain falls during 

 summer than winter in the middle latitudes. 



The following table exhibits a general view 

 of the relative depths of rain in different lati- 

 tudes : 



