As to the first of these, we may appeal to familiar facts within the personal knowledge of every man acquainted with the operations of sylvan nature. I have before me a letter from an acute and experienced observer, containing this paragraph: "I think that rain water does not ever, except in very trifling quantities, flow over the leaves in the woods in summer or autumn. Water runs over them only in the spring, when they are pressed down smoothly and compactly, a state in which they remain only until they are dry, when shrinkage and the action of the wind soon roughen the surface so as effectually to stop, by absorption, all flow of water." I have observed that when a sudden frost succeeds a thaw at the close of the winter after the snow has principally disappeared, the water in and between the layers of leaves sometimes freezes into a solid crust, which allows the flow of water over it. But this occurs only in depressions and on a very small scale; and the ice thus formed is so soon dissolved that no sensible effect is produced on the escape of water from the general surface. As to the influence of roots upon drainage, I believe there is no doubt that they, independently of their action as absorbents, mechanically promote it. Not only does the water of the soil follow them downward,[212] but their swelling growth powerfully tends to enlarge the crevices of rock into which they enter; and as the fissures in rocks are longitudinal, not mere circular orifices, every line of additional width gained by the growth of roots within them increases the area of the crevice in proportion to its length. Consequently, the widening of a fissure to the extent of one inch might give an additional drainage equal to a square foot of open tubing. The observations and reasonings of Belgrand and Valles, though their conclusions have not been accepted by many, are very important in one point of view. These writers insist much on the necessity of taking into account, in estimating the relations between precipitation and evaporation, the abstraction of water from the surface and surface currents, by absorption and infiltration--an element unquestionably of great value, but hitherto much neglected by meteorological inquirers, who have very often reasoned as if the surface earth were either impermeable to water, or already saturated with it; whereas, in fact, it is a sponge, always imbibing humidity and always giving it off, not by evaporation only, but by infiltration and percolation.