CLIMATIC CHANGES IN THE PRAIRIE REGION. 295 



entirely covered by water at an earlier period. The distinguished traveler made 

 the phenomena a subject of careful examination, proved the unsoundness of the 

 usually received hypothesis of a subterranean outlet, and did not hesitate to 

 explain the gradual depression of the level of the lake as the result of the numer- 

 ous clearings made in the valley during the latter half of the preceding century. 



"In 1822 Boussingault visited the locality and found that instead of retiring, 

 the waters of the lake had been sensibly rising for several years. A number of 

 sugar and cotton plantations on land formerly constituting a part of its bed had 

 become submerged, and the islands above the surface at the time of Humboldt's 

 visit had disappeared. No apparent reason existed for this, as no particular 

 change in the seasons had been noticed. Boussingault proceeds to state that 

 during the war for independence by the South American colonies, the fertile val- 

 ley of Aragua became the theater of bloody struggles, desolating its fields and 

 exterminating its population. The large plantations, which during the preceding 

 fifty years had been wrested from the domain of the forests were abandoned, and 

 in the tropical climate of Venezuela the ten or a dozen years that had elapsed had 

 been sufficient to cover them a second time with trees and shade ; the rise of the 

 water of the lake keeping pace with the encroachment of the forest. Other instan- 

 ces are mentioned by the same writer of large clearings in Venezuela and New 

 Grenada being accompanied by a similar disappearance of the waters of adjacent 

 lakes, while in districts always bare of trees, or where the forest had never been 

 disturbed, no such changes had occurred. This eminent scholar maintained that 

 the lakes of Switzerland have sustained a like depression of level since the too 

 prevalent destruction of woods and arrives at the general conclusion that 'in coun- 

 tries where great clearings have been made there has most probably been a dimi- 

 nution of the living waters which flow upon the surface of the ground, and that 

 very restricted local clearings may diminish and even suppress springs and brooks 

 without any reduction in the total quantity of the rain.' 



Marchand relates the circumstance of the Lome and the Doubs, streams in 

 Switzerland, from time immemorial furnishing an abundant water power for the 

 manufacturing establishments on their banks, becoming so deficient in the supply 

 of water after cutting the woods near their sources, as no longer to furnish the 

 required power, so that in one case steam had to be introduced and in the other 

 the factory was stopped entirely. 



Hummel mentions a striking instance of the influence of forests on the flow 

 of springs, as witnessed at Heilbroun. The woods on the hills surrounding the 

 town are treated, it appears, as a copse, being used only for supplying fuel. They 

 are cut at intervals of twenty or thirty years, and planted or allowed to shoot up 

 again from the roots. Regularly after each cutting the springs of Heilbroun fail. 

 But as the young shoots increase in size, the water flows more freely, and at length 

 bubbles up again in all its original abundance until the next cutting takes place. 



But the conservative influence of trees is not confined to mountainous and 

 warm countries. The climate of the Schelde Valley and the plains of Bavaria 

 and of Austria was so much injured by immoderate clearings, in the judgment of 



