Rain. 337 



with, circumstances. It will in general appear as a dry wind, 

 but may produce local showers, since it may act, through the 

 sudden addition of cold air, the part of a condenser. 



Forests are great generators of rain. This is mainly due 

 to the peculiar radiative power of trees and vegetables. The 

 soil, where it is covered with vegetation, receives no heat 

 directly from the sun, and but little through contact with the 

 heated air. It may seem like a confusion of cause and effect 

 to speak of vegetation-covered countries as rain-generators, 

 since abundant rain is so important a requisite for the abundant 

 growth of vegetables. This is, however, a case in which cause 

 and effect are interchangeable. E-ain encourages vegetation, 

 and vegetation in turn aids in producing a state of the super- 

 incumbent atmosphere, which encourages the precipitation of 

 rain. The result is that, apart from external agencies, regions 

 covered with abundant vegetation, and especially with high 

 trees, present year after year, and century after century, a 

 ranker and yet ranker luxuriance of vegetable growth. 



On the contrary, arid regions prevent, by their very 

 aridity, and consequently by the intense heat of the soil and 

 superincumbent air, the downfall of the showers which would 

 nourish vegetation. The result is, that even when the soil 

 itself is favourable, it is exceedingly difficult to convert an arid 

 into a vegetation- covered district, the want of moisture being 

 destructive to trees planted in such soils with the object of 

 encouraging rain-falls. The process of change must be a 

 gradual one. On the other hand, the improvement of a 

 region over which rain falls too heavily through overabundant 

 vegetation is a comparatively simple process, a judicious system 

 of clearing invariably leading to the desired result. 



The influence of the seasons remains yet to be mentioned 

 among the circumstances affecting the distribution of rain over 

 the earth's surface. The influence of the seasons is different 

 in different zones of the earth's surface. Under the tropics the 

 laws affecting the fall of rain are much more regular than else- 

 where. On the ocean we have clear skies where the trade- 

 winds are blowing steadily, and heavy rain falls by day over the 

 intermediate zone of calms ; but on the land we have regular dry 

 and wet seasons within the tropics. There is, properly speaking, 

 no winter or summer ; but applying these terms to the periods 

 at which winter or summer prevails in the temperate zones of 

 either hemisphere, we may say that the sky is serene in the 

 winter, becomes moist in spring, and the rainy season sets in 

 when the sun is near the zenith. Where there is a consider- 

 able interval between the sun's passages of the zenith, as in 

 places not very far from the equator, there are two wet seasons, 

 both occurring in summer. In countries in which monsoons 

 VOL. XII. — no. v. z 



