1849.] On the Influence of Forests on Climate. 793 



observation of the perspiration of leaves with this circumstance, Hum- 

 boldt observes properly, if a plant of such small size exercises in- 

 fluence upon evaporation, how much greater must be the perspiration of 

 the forests of the Upper Orinoco, which cover 2,60,000 nautical square 

 miles ! The cloudy and misty sky of those regions, and of the 

 Province of Las Esmeraldas, to the west of the Volcano of Pichinche, 

 the decrease of the temperature in the missions on the Rio Negro, and 

 the streams of vapour which become visible on fixing the eyes on the 

 tops of the trees in the Equatorial forests, must be alike ascribed to the 

 aqueous exhalation of the leaves and to their radiation towards the 

 space of the atmosphere. * * * * * * 



" It is asserted that there is at present much less rain in Barbadoes 

 than there was formerly, and many of the inhabitants ascribe it to 

 the unlimited clearance of forest and brushwood, and although we have 

 no direct reasons to prove why such clearances lessen the annual quan- 

 tity of rain, we have abundant proof that it is so. In every instance 

 and in every part of the globe where forests have been cleared, a dimi- 

 nution of aqueous precipitations has been noted ; and as it is a fact 

 which remains uncontested, that Barbadoes, within the last fifty years 

 was much more wooded than it is now, the diminution of rain must 

 likewise be expected as the natural effect. The evidence of Humboldt, 

 Leopold de Buck, Daniell, Dove, and others, is so powerful on this 

 subject, that I should wish to press particularly upon the attention of 

 the reader how important the existence of wooded spots become to the 

 agriculturist. I cannot do better than quote the words of Humboldt 

 to enforce this view : — '* By felling the trees that cover the tops and 

 the sides of mountains, men in every climate prepare at once two 

 calamities for future generations — the want of fuel and a scarcity of 

 water; trees, by the nature of their perspiration, and radiation from 

 their leaves in a sky without clouds, surround themselves with an 

 atmosphere constantly cool and misty." 



Again, that forests exist in those parts only where the predominant 

 winds carry with them a considerable quantity of moisture, and con- 

 sequently that they are not the primary cause of humidity, is rendered 

 highly probable from the following consideration : — 



" *In all countries having a summer heat exceeding 70° the pre- 



* Maclaren, Art. America. Encyc. Britannica. 



5 K 



