228 LIGHT SCIENCE FOR LEISURE HOURS. 



exhibited in a hundred striking and interesting physical 

 relations. What, for instance, can be stranger or more 

 poetical than the contrast which Professor Tyndall has 

 instituted between that old friend of the agriculturist 

 tho wintry snow-flake and the wild scenery of the 

 Alps? 'I have seen,' he says, 'the wild stone-ava- 

 lanches of the Alps, which smoke and thunder down 

 the declivities with a vehemence almost sufficient 

 to stun the observer. I have also seen snow-flakes 

 descending so softly as not to hurt the fragile spangles 

 of which they were composed; yet to produce from 

 aqueous vapour a quantity which a child could carry 

 of that tender material demands an exertion of energy 

 competent to gather up the shattered blocks of the 

 largest stone-avalanche I have ever seen, and pitch 

 them to twice the height from which they fell.' 



I may point out in this place the important con- 

 nection which exists between the rainfall of a country 

 and the amount of forest land. I notice that in 

 parts of America attention is being paid with mark- 

 edly good results to the influence of forests in encou- 

 raging 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 tends to produce a state of the super- 

 incumbent atmosphere which encourages the preci- 

 pitation of rain. The consequence is, that it is very 

 necessary 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 



