INFLUENCE OP FORESTS ON CLIM4TE. 75 



1865, for March ISth to Nov. 14th— 8 months— give 25.35 inches 

 rain-fall, and 30.85 inches evaporation. For 1866, rain-fall for 8 

 months 29.18 inches, evaporation 32.03 inches. Observations 

 made at Milwaukee, Wisconsin, extending through five years, and 

 taken from March 15th to Nov. 14th — 8 months of each year, the 

 average rain 23.61 inches, while the average evaporation for the 

 same period was 32.58 inches. In the open country where drying 

 winds prevail and much land is exposed by tillage, evaporation 

 may take place to the extent of three-fourths of the rain-fall 

 throughout the year, or more than twice that fall for the entire 

 summer. Hence the value of forests as arresters of evaporation, 

 or as bai-riers against the sweep of drying winds, becomes obvious. 



Observations near London, England, show a mean evaporation 

 of 19.11 inches; and at Manchester of 25 inches. "The chief 

 cause of the difference in dryness between the United States and 

 England, may be found in the. fact that the humidity is there borne 

 from the ocean, while the .prevailing west winds bear our land 

 moisture away from us towards the sea, drying us, instead of 

 increasing our store of vapor." 



Researches into the phenomena of heat have disclosed the extra- 

 ordinary fact that vapor of water is opaque to the rays of heat of 

 low intensity, such as that which proceeds from the soil and from 

 plants by night ; in other words, that the heat of the earth cannot 

 be i-adiated or projected towards the sky, if there exists in the 

 air above the spot observed a large proportion of aqueous vapor. 

 Through pure, dry air, the heat may pass off as readily as if no 

 air there existed. It has been calculated that of the heat radiated 

 from the earth's surface, warmed by the sun's rays, one-tenth is 

 intercepted by the aqueous vapor within ten feet of its surface. 



Hence the powerful influence of moist air upon climate. Like a 

 covering of glass, it allows the sun's rays to reach the earth, but 

 prevents, to a great extent, the loss by radiation of the heat thus 

 communicated. In accordance with this theory, is the fact that 

 the withdrawal of the sun from any region over which the atmos- 

 phere is dry, is followed by quick refrigeration. On the elevated 

 plains of central Asia, "the winters are rendered almost unendur- 

 able from an uninterrupted outward radiation, unimpeded by 

 aqueous vapor." 



Professor Tyndallsays, "The removal for a single summer night 

 of the aqueous vapor from the atmosphere that covers England, 

 would be attended by the destruction of every plant which a freez- 



