11 



to a well-wooded country, that a very marked change occurs ; in the woods 

 the air is more nearly saturated with vapor, the ground is moist, and not 

 hardened by the loss of water , and hence no furthef arguments need be ad- 

 duced to show that clearmg away the forests from d coimtry will increase iJie dry- 

 ness of the ground. 



This effect of aqueous vapor in preventing the radiation and loss of heat 

 from, the ground is subject to direct measurement. Buring nine days in 

 June, 1866, at Milwaukee, when the air was most nearly saturated with 

 vapor (relative humidity 90 per cent.), it was found that the difference of 

 temperature of the day, and of the night was 18o, — while in the same number 

 of days when the air was most dry (relative humidity being only 56 per cent.), 

 the difference of temperature between day and night was 29o, showing a dif- 

 ference of 110, due to the existence of the vapor, which acts like a blanket 

 in preventing the occurrence of frosts and the consequent destruction of 

 tender plants. Forests, by increasing the moisture of the air adds to the 

 security against frosty nights. 



From observations made at the agricultural college at Lansing, Michigan, 

 by Dr. K. 0. Kedzie, as compared with those made at Milwaukee, it appears 

 that the amount of evaporation from the surface is considerably greater in 

 the latter place ; and in the interior of Wisconsin, away from the influence 

 of the lake, the difference must be still greater — ^thus showing a greater neces- 

 sity for the protection by forest trees in this state than in Michigan. This 

 necessity continues to increase as we go westward until we approach the arid 

 and almost desert plains at the foot of the Rocky Mountains. 



Trees have a very decided effect upon the temperature and humidity of the 

 air by the evaporation constantly going on from the surface of the leaves. It 

 is a principle in chemical science, that evaporation produces cold ; and care- 

 ful experiments show, that to convert water into steam or vapor 965o of heat 

 must be supplied. In the steam engine this supply comes from the fuel con- 

 sumed for the purpose, but in ordinary evaporation an equal quantity comes, 

 (though a longer time is required,) from the surrounding air. To absorb heat 

 from any medium is to make it cold ; and the amount of heat absorbed from 

 the atmosphere by the evaporation of water from leaves of trees must there- 

 fore be very considerable. 



To estimate roughly, but with as much accuracy as possible, the vast quan- 

 tity of water consumed by vegetation in one year we may assume the follow- 

 ing data : 



The average amount of earthy matter in plants, or the amount of ash left 

 when consumed by fire is about five per cent , or one pound in every twenty. 

 The average amount of earthy matter contained in natural waters is about 

 eight grains* in a gallon (of 58,818 grains) or one pound in 7,290 pounds. 

 From this it follows, that to obtain 20 pounds of wood, or other vegetable 



* The water of Lake Michigan has recently been found to contain 8.30 grains in a gallon. 



