I30 TEE POPULAR SCIENCE MONTHLY 



whicli were thought to have resulted therefrom. However, the aurora 

 has not yet been satisfactorily explained. With the exception of the 

 aurora, there is no known relation between terrestrial magnetism and 

 atmospheric phenomena. 



The question as to whether or not forests affect weather and climate 

 has been much debated. Eecent investigations have brought out 

 the following facts: Whatever influence forests have upon meteoro- 

 logical conditions is purely local, and even that influence is not marked. 

 In one case it was found that the mean annual temperature within a 

 forest was only a few tenths of a degree cooler than at a point a half 

 mile or a mile outside the forest border, the greatest difference amount- 

 ing to 2° F. The relative humidity was at times 7 per cent, greater 

 within the forest than in the open country. In the United States the 

 wholesale destruction of forests, which has been going on since colonial 

 times, has not been accompanied by any marked increase or decrease in 

 rainfall. On the other hand, the reforestation of large tracts in central 

 Europe and in northern Africa during the past century has not resulted 

 in an appreciable effect upon the precipitation observed during that 

 period. Forests are the effect rather than the cause. There is still 

 considerable confusion in the public mind concerning rainfall and flow- 

 off, when the supposed influence of forests is considered. Deforestation 

 has undoubtedly increased the frequency and the intensity of floods in 

 small constricted districts, notably in certain mountain valleys, but 

 where the removal of the forest cover over large areas has been followed 

 by cultivation of the soil the rate of flowoff has remained unchanged. 

 Prom hydrogi'aphs of the principal rivers of the United States it is 

 apparent that liigh waters are neither higher nor low waters lower than 

 they were fifty years ago, and they are neither more frequent nor of 

 longer duration now than they were then. N'otable floods like that of 

 Paris, France, during the spring of 1910, and that of the Ohio Valley in 

 the spring of 1913, are the result of a number of causes, in which the 

 excessive rainfall was in no way related to the presence or absence of 

 forests, and in which the rapid flowoff was more dependent upon the 

 frozen soil than upon the recent removal of the forest cover. That the 

 flowoff is more rapid when the ground is frozen explains the greater 

 frequency of floods during spring than during any other season of the 

 year. Moreover, forests tend to preserve the snows of winter, as well as 

 to retain the fertile elements of the soil from washing away. Wliile 

 forests are thus of importance to the agriculturalist and the engineer, 

 they are of little concern to the student of the weather. 



The deep-seated notion, held by many individuals, that the climate 

 is changing is often referred to in expressions like "old-fashioned 

 winter," " the storms we used to have," and " the deep snows when 

 I was a boy," etc. Subjective phenomena like these are of inter- 



