792 On the Influence of Forests on Climate. [Aug. 



to be diminishing on account of the rapid disappearance of the woods 

 in the interior, when government had recourse to the measure of pro- 

 hibiting their further destruction, and they rapidly recovered their 

 former dimensions. In fact in all tropical countries, where the quan- 

 tity of aqueous vapour in the atmosphere is great, but where on the 

 other hand, the direct rays of the sun are most powerful, any impedi- 

 ment to the free circulation of the air, or any screen which shades the 

 earth from the solar rays, becomes a source of humidity, and wherever 

 dampness and cold have begun to be generated by such causes, the 

 condensation of vapour continues. The leaves moreover of all plants 

 are alembics, and some of those in the torrid zone have the remarkable 

 property of distilling water, thus contributing to prevent the earth 

 from becoming parched. 



But there are various circumstances which may contribute to- 

 wards the formation of rain, and to which I have alluded in the pre- 

 ceding remarks ; temperature, pressure of the atmosphere and its elec- 

 trical state, are the chief agents ; mountain chains and forests form local 

 causes. 



The effect which forests exercise upon the condensation of vapours 

 has been ably treated by Daniell, in his Meteorological Essays. 



" Humboldt considers that forests exercise a triple influence upon 

 climate — first they protect the soil against the rays of the sun ; secondly, 

 they produce, by the vital activity of their leaves, a constant evaporation 

 of aqueous vapours ; thirdly, these leaves increase the radiation. These 

 three simultaneous causes, as affording shade, evaporation, and radiation, 

 are so influential that the knowledge of the extent of forests compared 

 with the naked savannahs, steppes and champaign ground, forms one 

 of the most important elements in the climatology of a country. The 

 active vitality of plants consists chiefly in the leaves ; they are the 

 organs of respiration, digestion and nutrition. The great quantity of 

 water which they perspire may be easily proved by placing a glass next 

 the under-surface of a young vine leaf on a hot day, and it it will be 

 found to perspire so copiously, that the glass will be in a short time 

 covered with dew, which runs down in streams in half an hour. Hales 

 computed the perspiration of plants to be seventeen times more than 

 the human body ; he calculated that the leaves of a single helianthus, 3 

 feet and \ in height, covered 40 square feet, and comparing his former 



