136 OUTLINES OF FORESTRY. 



" 2. What influence do the forests exert, through the absorp- 

 tion of their roots or the evaporation of their leaves, in modi- 

 fying the hygrometrical conditions of the surrounding atmos- 

 phere ? 



" 3. How do they modify the temperature of a country ? 



" 4. Do the forests exercise an influence on the amount of 

 water falling, and upon the distribution of rain throughout 

 the year, as well as upon the regulation of running waters and 

 springs ? 



" 5. In what manner do they intervene in the preservation of 

 mountains and slopes ? 



" 6. Do the forests serve to draw from the storm-clouds their 

 electricity, and by thus doing diminish their effects upon the 

 neighboring regions not wooded ? 



" 7. What is the nature of the influence that they may be 

 able to exercise upon the public health ? 



" From these questions we may see what questions we must 

 solve before being able to decide as to the influence that the 

 clearing off of woodlands may exercise upon the climate of a 

 country." 



Much valuable data concerning the results of 

 the destruction of the forests on climate, rainfall, 

 and other meteorological conditions, were collected 

 by several scientific expeditions sent to Brazil 

 under the direction of Louis Agassiz. 



In an account of the Thayer expedition in 1865 

 and 1866, Professor Hartt, in a description of the 

 " Geology and Physical Geography of Brazil," * 

 page 319, thus refers to the marked effects that have 



