26 Forests — their Itijluence upon Climate and Rainfall. [January, 



season absorbed by the myriad root hairs, and partly also to 

 the circumstance that the vegetable mold usually carpeting the 

 soil of the forest is well qualified to soak up water and prevent its 

 running off too rapidly through superficial channels. 



The humbler specimens of vegetation also have an effect, as is 

 conclusively shown by the following experiment : A pot with 

 artificially prepared soil, similar to that used in the above experi- 

 ments, was used. Another vessel of the same size and weight in 

 which grass ( Poa annua) about four inches high was growing, 

 was also employed. Now it was found by repeated testing that 

 from the pot containing the grass the evaporation exceeded that 

 of the pot having only soil. The rates in ounces would be about 

 five to four for the grass and soil respectively. 



From all these investigations the writer is able to confirm his 

 former investigations in regard to transpiration, 1 and in these ex- 

 periments it was particularly observed that while the evaporation 

 from the soil was greatly influenced by temperature and the de- 

 gree of humidity — for the mean temperature and dew point were 

 both noted in all these experiments — transpiration was excited to 

 a greater degree by the direct rays of the sun. 



From the data just obtained it would seem safe to infer that 

 when the percentage of woodland is fair (25 to 30 per cent.) at 

 least twelve inches of water is transpired in the course of a sea- 

 son in mild or temperate climates, or, in other words, twelve 

 inches of the total annual terrestrial evaporation. All this vast 

 amount of water is transpired in about six months, or during the 

 vegetative period. Under these circumstances an equivalent of 

 nearly half the rainfall during the warm season may be accounted 

 for by the transpiration. These are striking facts, and tell in in- 

 disputable terms of the happy effect of plant life upon the humid- 

 ity of our atmosphere, as this substance in due proportion is very 

 essential to an equable and salubrious climate. Were it not that 

 the atmosphere was properly moistened so as to intercept noc- 

 turnal radiation from the earth, our cereals and other products of 

 husbandry as well as vegetation generally, would greatly suffer if 

 not be entirely destroyed by the resulting frost. 



It is also a noteworthy fact that the exhalation of moisture 

 from the vast surface presented by the leaves is nearly constant 

 even during long droughts; and when streams and shallow waters 



1 Transpiration of Plants, American Naturalist for March, 1878. 



