308 SOILS. 



Dew rarely adds Moisture. Candid observations will con- 

 vince any one, therefore, that in most cases the supposed addi- 

 tion to the moisture of the soil from dews is an illusion. 

 Whatever dewdrops fall on the ground are in general simply 

 the return to the soil of a part of what came from it; while 

 the dew that evaporates from the bedewed leaves or other 

 objects represents simply a delayed outgo of moisture from 

 the soil, which for a time retards evaporation direct from the 

 soil, and thus effects a slight saving of moisture. 



But while this is measurably true of inland and especially of continental 

 areas like the great plains of North and South America, it is also true 

 that in deep moist valleys, and on the rainy and foggy coast regions of 

 continents, dews are found to both fall and rise, not uncommonly to 

 such an extent as to be equivalent to a not inconsiderable aggregate 

 precipitation. Thus in the moist coast belt of Oregon and Washington 

 lying west of the Cascade range of mountains, the morning dews of 

 summer are frequently so copious that the water falls in showers from 

 the lower trees and shrubs, so as to necessitate the use of water-proof 

 clothing when traversing the woods in the morning, quite as much as 

 though rain was actually falling. In hilly and more especially in 

 mountainous regions the cold air descending from above and flowing 

 down in the ravines will often cause a heavy condensation of dew in 

 these, while the bordering ridges, which rise above the cold currents, 

 remain free from dew. These descending currents as a rule not only 

 bring no surplus moisture with them, but in their downward course 

 become warmer by contraction and therefore relatively drier. In these 

 cases also, therefore, the dew is purely moisture derived from the 

 ground, which in rising encounters the cold air and is thus condensed. 



The fact that dew is most commonly derived from the soil 

 could have been foreseen from the other fact, long ascertained 

 and known, that during the night the soil is as a rule warmer 

 than the air above it; as has been shown by the earlier ob- 

 servers, as well as more specifically by Stockbridge. 



Dew 'within flic Soil. It is obvious that whenever dew is 

 formed above the surface of the soil, the air within the latter 

 must be at or near its point of saturation with vapor, as in 

 fact is usually the case a few inches below the surface. It 

 follows that when a depression of temperature occurs within 



