630 SCIENCE PROGRESS 



examination of the views held at different times leaves little 

 doubt that they varied directly with cycles of meteorological 

 conditions. 



The difficulties of discriminating between cause and effect 

 render it advisable to restrict any conclusions as to the influence 

 of forest to what is proved beyond doubt. The mean annual 

 temperature is found to be slightly lower (about "i to i° C.) in 

 forest than in the open country and the extremes both of 

 heat and cold are reduced. No difference is to be found in the 

 absolute humidity of the air within and without forest, so that 

 only the somewhat lower temperature in forest gives rise to 

 greater relative humidity. 



This leads at once to the increase in rainfall due to forest 

 which is popularly supposed to be considerable, and in which 

 some people, not unnaturally, see a great drawback to affores- 

 tation in the United Kingdom. Experiments are still being 

 made with a view to gauging to what extent this is the case. 

 An increase is probably owing to the higher relative humidity 

 in forest and the fact that the presence of the trees forces the 

 clouds to rise, and that the trees with their branches and foliage 

 mechanically sift the moisture-laden atmosphere. 



On the other hand, air currents are constantly reducing the 

 excess in relative humidity in the comparatively small volume 

 of air within the forest, and, comparing the height of forest trees 

 with the extent of atmosphere in which precipitation originates, 

 the mechanical influence is not likely to be very noticeable. 



In short, experiments have so far shown a slight increase in 

 precipitation extending only a very short distance beyond the 

 forest's limits : the influence ascribed to the presence of forest 

 is more often than not due to the configuration of the ground, 

 forests being now largely restricted to hilly districts unsuitable 

 for agriculture. All fears, then, of a considerable increase of the 

 rainfall of the United Kingdom owing to afforestation may be 

 dismissed as unfounded. 



Other influences of forest in the circulation of moisture are 

 much more marked. Only a part of the precipitation actually 

 reaches the soil, whilst a larger quantity of water is absorbed 

 by trees than by crops or grass; this is counteracted to some 

 extent by less evaporation of water directly from the soil ; but, 

 speaking generally, the fact that forest soil is only moister down 

 to six inches in depth and drier than that of other land at a 



