THE FORCING POWER OF RAIN. 229 



become excessively dry, it is almost impossible to 

 remedy the mischief, simply because the want of 

 moisture is destructive to the trees which may be 

 planted to encourage rainfall. Thus there are few 

 processes more difficult (as has been found by expe- 

 rience 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 diffi- 

 culty. 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 

 unclouded condition of our skies. I believe that, on 

 the whole, the skies have been rather more cloudy 

 than usual this year. The fact that so little dew has 

 fallen is a sufficient proof that the nights have been on 

 the whole more cloudy than usual, since, as is well 

 known, the presence of clouds, by checking the radia- 

 tion of the earth's heat, prevents (or at least diminishes) 

 the formation of dew. The fact would seem to be that 

 the westerly and south-westerly winds which usually 

 blow over England during a considerable part of the 

 year, bringing with them large quantities of aqueous 

 vapour from above the great Gulf Stream, have this 

 year blown somewhat higher than usual. Why this 

 should be it is not very easy to say. The height of 



