164 EFFECTS OF FORESTS ON RAINFALL. 



rainfall, and what is thus certified is in accordance with all that has 

 been advanced in preceding chapters. 



Rain has been spoken of as the precipitation from the air of 

 moisture in excess of what could be sustained in solution at the 

 temperature at the time. The increased moisture passed into the 

 atmosphere by evaporation through the stomates of the leaves must 

 render the air surmounting and surrounding a forest more liable to 

 be affected by any change of wind than air in the open field. This 

 it will do irrespective of any other means whereby the quantity of 

 moisture in the air there may be increased; and thus may the increase 

 of rainfall in question be accounted for. 



There may alcso be an increase of rainfall occasioned by the repeated 

 evaporation and precipitation of the same moisture : it is not the 

 source of the moisture but the measure of the precipitation occurring 

 in the locality which is alone in question here. 



Besides this, it is alleged, and I believe correctly, that the heated 

 air passing over a forest becomes cooled, and may thus be led to 

 deposit the moisture with which it is charged. 



While it is thus that trees generally act in maintaining the 

 humidity of the district in which they grow, it is not impossible that 

 they may act also in a way much more like what may be called 

 attracting rain, by their acting as lightning conductors. 



If a small pan with a perforated bottom be filled with water, and 

 suspended from the prime conductor of an electrical machine, on the 

 cylinder being put in motion, the water, instead of slowly dropping 

 as it did before, pours in torrents through the perforations in the 

 bottom of the pan, an electric attraction as well as an atti-action of 

 gravitation drawing it downwards. And this experiment has been 

 employed to illustrate the thunder shower. Tapering points are 

 found to have the effect of drawing off a surcharge of electricity from 

 a body towards which they are directed ; and thus, it may be, do 

 trees occasionally act as lightning conductors, attracting the electric 

 fluid, and thus bringing the water charged with it. 



Observations have been reported to me which are in accordance 

 with this supposition. 



The Rev. J. Thomas, of Capetown, writes to me that when occupy- 

 ing a station of the Wesleyan Missionary Society on the West Coast 

 of Africa, on one occasion when on a journey from the interior in 

 1848, he and his fellow-travellers came one evening to a forest where 



