224 PHYSICAL PROPERTIES OP SOIL. 



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 wood- 

 land was determined 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 3 1-2 in. 

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

 the leaves carefully counted, were in number, 21,192. 

 Supposing these all to have evaporated like those in 

 the bottle, they would have expired, in twelve hours, 

 339,072 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, 3,875 gallons of 

 water, or 3 1 ,000 lbs. were evaporated from an acre of 

 woodland in 12 hours. At Rutland, in Vermont, 

 where this experiment was made, in 1789, the Pro- 

 fessor notes, that on the 26th of May, the maple- 

 leaves were 1-6 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 



