252 LIGHT SCIENCE FOR LEISURE HOURS. 



and the amount of forest-land. "We notice that in 

 parts of America attention is being paid with mark- 

 edly good results to the influence of forests in encour- 

 aging rainfall. We have here an instance in which 

 cause and effect are interchangeable. Rain encourages 

 the growth of an abundant vegetation, and abundant 

 vegetation in turn aids to produce a state of the super- 

 incumbent atmosphere which encourages the precipita- 

 tion of rain. The consequence is, that it is very neces- 

 sary to check, before it is too late, the processes which 

 lead to the gradual destruction of forests. If these 

 processes are continued until the climate has become 

 excessively dry, it is almost impossible to remedy the 

 mischief, simply because the want of moisture is de- 

 structive to the trees which may be planted to encour- 

 age rainfalls. Thus, there are few processes more 

 difficult (as has been found by experience in parts of 

 Spain and elsewhere) than the change of an arid region 

 into a vegetation-covered district. In fact, if the region 

 is one of great extent, the attempt to effect such a 

 change is a perfectly hopeless one. On the other hand, 

 the contrary process that is, the attempt to change a 

 climate which is too moist into one of less humidity 

 is in general not attended with much difficulty. A 

 judicious system of clearing nearly always leads to the 

 desired result. 



The dryness of the past year has not been due to 

 the want of moisture in the air, nor to the exceptionally 



