PHYSICAL PROPERTIES OF SOIL. 247 



an acre of fresh-ploughed land is equal to 950 lbs. per hour ; 

 this is the greatest for the first and second days, ceases about 

 the fifth day, and again begins by hoeing, while at the same 

 time the unbroken sod affords no trace of moisture. This 

 evaporation is equal to that which follows after copious rains. 

 These are highly practical facts, and teach the necessity of 

 frequent stirring of soil in a dry time. Where manure or 

 geine is lying in the soil, the evaporation is from an acre 

 equal to 5000 lbs. per hour. At 2000 lbs. of water per 

 hour, the evaporation would amount in 92 days to 2208000 

 lbs. which is nearly equal to the amount of rain which would 

 fall in the same time in this climate. But the evaporation 

 from woodland actually exceeds the amount of rain which 

 falls. The evaporation from an acre of woodland was deter- 

 mined by Professor Williams, (see his Hist, of Vermont, 

 vol. I.,) as follows : two leaves and a bud of a branch of a 

 growing maple were sealed in a bottle, while yet attached 

 to the tree. The expired water collected and weighed, was 

 found to amount in six hours to 16 grains. The tree was 8| 

 niches in diameter, and 30 feet high. It was felled, and the 

 leaves carefully counted, were in number, 21192. Suppos- 

 ing the^ all to haye evaporated like those in the bottle, they 

 M'ould have expired, in twelve hours, 339072 grains of water. 

 A moderate estimate, and below the usual quantity of wood 

 per acre of similar land, gave four such trees to a rod, or 

 640 per acre. Estimating 7000 grains to a pint, 3875 gal- 

 lons of water, or 31000 lbs. were evaporated from an acre 

 of woodland in 12 hours. At Rutland, in Vermont, where 

 this experiment was made, in 1789, the Professor notes, that 

 on the 26th of May, the maple leaves were ^ of their full size, 

 and on the 15th of September following, these leaves began 

 to turn white. Throwing out the 15 days in September, and 

 the 4 in May, the leaf may be considered as fully developed 



