EFFECTS OF DESTRUCTION OF FORESTS. 411 



small particles, by what is commonly called evaporation. How- 

 ever dry the atmosphere may appear to be, it is always more or 

 less filled with insensible moisture. If during the greatest heat of 

 summer one fills a pitcher with ice-water, the outer surface of the 

 pitcher will be immediately covered with drops of water. This is 

 nothing more than the condensation of the insensible moisture, and 

 is produced by the coldness of the ice-water. Now, the moisture 

 taken up by the evaporating force of the sun's rays, not only from 

 the oceans and rivers, but from the surface of the solid earth, is 

 immense in amount; and according to its degrees of density is 

 seen in the form of clouds. These clouds are light when compared 

 with the air, and float in it at different heights, according to their 

 difiTerent degrees of rarification, and are visible or invisible in pro- 

 portion to their density. 



Now, again, these clouds, which are really water in a vaporous 

 state, are carried by the winds, and distributed everywhere above 

 the surface of the earth ; and in order to become rain, and to fall 

 in showers, certain conditions of temperature are necessary. 

 Clouds are of two kinds, the cumulus and the cirrus, — that is, the 

 first are formed of those vapors which at a certain height become 

 vesicular, and the second of those which become frozen. -All other 

 clouds are merely modifications of these two. There is no doubt 

 but that electricity has a great influence in the production of 

 storms ; but this element may at present be dispensed with, as it 

 is desirable to present only the simplest idea of a rain-cloud. The 

 two abovementioned kinds of clouds then are supposed to be abso- 

 lutely necessary, to produce the showers" and that it cannot take 

 place without their conjunction and cooperation. When, therefore, 

 the vesicular vapors come in contact with the frozen vapors, the 

 instant result is condensation and the consequent rainfall. 



We must now mention two other conditions afiecting the form- 

 ation of rain, or at least its occurrence in certain localities. These 

 are the cold and the warm radiations from the earth. The warm 

 radiations take place from level and sandy, treeless plains ; and 

 according to the theory which we shall endeavor to prove, there 

 should be but few or no rainfalls upon them. On the other hand, 

 the cold radiations rise from high wooded mountains, and heavily 

 wooded plains. According to the theory again, there should be 

 frequent and abundant rains in these localities. It is but fair in 

 this connection to state, however, that there is a variety of opinions 

 with regard to cold radiations, among scientific men ; and also 



