TILE DBAIHAGE. 17 



in three ways : It removes the surplus water which other- 

 wise, frozen in winter, must both thaw and evaporate in 

 spring, both of which are chilling processes ; second, it keeps 

 the air-spaces open, and warm air ascends in winter from 

 the subsoil, always warmer in winter than the soil. Third, 

 through these open air-spaces warm showers soak down in 

 spring and warm the soil. 



As to the fiist point, thawing and evaporation both keep 

 the soil and the adjacent atmosphere colder. Thawing ice, 

 especially by heat from above, is a slow process. Let the 

 water freeze solid in a wooden pump, and try to thaw it 

 with hot water from above, and you will be convinced. In 

 spring the ice on and in an undrained soil must be thawed at 

 an immense waste of sun-heat which would, if the land were 

 drained, be used in warming and drying the soil, and in 

 germinating seeds, in that case already sown. This thawing 

 in spring takes the warmth out of the air just as the ice in 

 an ice-cream freezer, melting by the chemical action of the 

 salt, requires warmth and takes it from the cream, freezing 

 the latter. We all know, too. how chilly the air is when 

 snow and ice are thawing in the fields in spring time. Thus 

 the mere thawing of the ice on and in a saturated clayey soil 

 may delay the starting and growth of crops a full week or 

 more in spring time. But, now, suppose it all to be melted 

 at last. It must still be evaporated from above (nearly all of 

 it in a tenacious undrained clay soil) by the sun's heat 

 before plowing, planting, growth, and tillage can begin ; 

 and evaporation is not only a very slow process, but a very 

 cooling one. Wash your hands, even in warm water, out- 

 doors in a brisk wind (where evaporation is rapid), and hold 

 them up in the wind and you will see how quickly they 

 are chilled. We sprinkle pavements and porch floors in 

 summer, and evaporation cools the air. Increased evap- 

 oration cools the air after a summer shower. Boys coming 

 out from swimming into a brisk wind are chilled by the 

 evaporation from their wet bodies. 



