THE CLIMATIC INFLUENCE OF VEGETATION. 389 



Forests produce rain. Under the influence of vertical sun-rays trees 

 exhale the aqueous vapors which their leaves linve absorhed from the 

 atmosphere, and in contact with tlie night-air or any stray current of 

 lower temperature, these vapors discharge rain-showers even in mid- 

 summer, and at a great distance from the sea. 



By moistening the air woodlands also moderate the extremes of 

 heat and cold. It is seen on the sea-shore how beneficently humidity 

 operates in allaying the severity of winter, and in summer the evap- 

 oration of dew and rain gives us cool breezes when they are most 

 needed. By the extirpation of forests the climate of the entire OrMs 

 Homaniis has been changed from the summer temperature of West 

 Virginia to the furnace-heat of New Mexico and Arizona. 



Besides this, the forest by shade in summer and fuel in winter 

 protects us directly against the vicissitudes of temjierature, and at the 

 foot of high mountains interposes a mechanical barrier between the 

 valleys and avalanches in the north, and floods in the south. The 

 water-torrents, which not only flood and damage the lowlands, but 

 carry their fertile soil away, are imbibed or detained by extensive 

 forests. Joseph II. of Austria was right to attach heavy penalties 

 to the destruction of the " Bannwalder," the woods on the Alpine 

 slopes, that protect the valleys from avalanches, and to propose that 

 in wars, even d Poutrance, the trees of a country should be spared by 

 international agreement. 



Our woods are also the home and shelter of those best friends of 

 man, the insectivorous birds. A country destitute of trees is avoided 

 by birds, and left to the ravages of locusts and other insects, which, 

 as we saw on our own continent, always attack the barren and naked 

 districts. Our locust-swarms devastated the " Great "West," i. e., the 

 treeless expanse between the Mississippi and the Rocky Mountains, 

 but spared the woodlands of the Alleghanies and the timber-regions 

 of the Pacific slope. 



The exhilarating influence of a woodland excursion is not alto- 

 gether due to scenic effects and imagination. Forests exhale oxygen, 

 the life-air of flames and animal lungs, and absorb or neutralize a 

 variety of noxious gases. Scirrhous affections of the skin and other 

 diseases disappear under tlie disinfecting influence of forest-air. Dr. 

 Brehm observes that ophthalmia and leprosy, which have become 

 hereditary diseases, not only in the valley of the Nile, bvit also on the 

 table-lands of Barca and Tripoli, are utterly unknown in the well-tim- 

 bered valley of Abyssinia, though the Abyssinians live more than a 

 hundred geographical miles nearer to the equator than their afflicted 

 neighbors. 



The traditions of the "blessed islands of the West," the " Garden 

 of the Hesperides," probably referred to Madeira and the Cape- Verde 

 Archipelago, which, according to De Gama's description, must have 

 come nearer to our idea of terrestrial paradise than any other region 



