RAIN. 105 



The forests should, therefore, be preserved on the 

 mountain-sides, in order to protect the lowlands 

 either from inundations or floods, or from the 

 effects of too small a quantity of water in the 

 rivers which flow through them during droughts. 



The action of mountains in cooling the air and 

 causing the condensation of the moisture of the 

 air, is thus referred to by Tyndall in his " Heat as 

 a Mode of Motion," * page 384 : 



" Mountains act as condensers, partly by the coldness of 

 their own masses, which they owe to their elevation. Above 

 them spreads no vapor screens of sufficient density to intercept 

 their heat, which, consequently, passes unrequited into space. 

 When the sun is withdrawn, this loss is shown by the quick 

 descent of the thermometer. The difference between a ther- 

 mometer which, properly protected, gives the true tempera- 

 ture of the night-air, and one which is permitted to radiate 

 freely towards space, must be greater at high elevations than 

 at low ones. This conclusion is confirmed by observation. 

 On the Grand Plateau of Mont Blanc, for example, MM. 

 Martins and Bravais found the difference between two such 

 thermometers to be twenty-four degrees Fahrenheit, when 

 a difference of only ten degrees was observed at Chamouni." 



* Keprinted, by permission, from " Heat as a Mode of Mo- 

 tion," by John Tyndall, LL.D., F.R.S. New York : D. Apple- 

 ton & Co., 1883. Pp.591. 



