18 TILE DBAINAGE. 



Many other illustrations might be given of the fact that 

 both thawing and evaporation are very cooling processes. 

 Undrained clay soils are called kt cold soils," and sandy 

 loams are called u warm soils," partly on account of this 

 thawing and this drying by evaporation, that must take 

 place on the former. But tile-draining a clayey soil saves 

 these two wastes of heat, and makes it warm earlier in 

 spring and warmer by several degrees than the vmdrained 

 adjacent soil all summer, as has been shown by careful tests 

 with thermometers placed in both soils. 



The second way in which underclrainage, natural or arti- 

 ficial, warms the soil is by keeping the proper air-spaces (the 

 larger pores) open in the soil, and not full of hydrostatic 

 water, or ice in winter. When thus open, the warmth from 

 the warmer subsoil ascends through them and helps to warm 

 the soil. Suppose the soil is frozen But the subsoil five 

 feet deep will be about 50 warm, and will help thaw the 

 frozen soil, which is about 20 cooler. If the air-spaces are 

 open this warmth readily ascends. 



The third point under this fifth general head was, that 

 tile drainage keeps the pores, or air-spaces, open for the de- 

 scent of the warm rains of spring and summer, and these 

 carry down their warmth into soil and subsoil, liain can 

 not descend through the small capillary spaces. They are 

 full of moisture held up to or near the surface by capillary 

 attraction. But in the larger, proper air-spaces, capillarity 

 does not oppose gravitation, and the warm rain sinks rapid- 

 ly and warms the soil. 



As to frost action, it is a curious and seemingly paradox- 

 ical fact that porous soils freeze deeper in winter, though 

 they thaw earlier in spring. Frost goes down faster when 

 the air-spaces are open, just as warmth goes both down and 

 up more readily, as has been seen. If you are skeptical on 

 this point, examine the two kinds of soil described, or try 

 the following simple experiment in winter time. Take two 

 large sponges, as nearly alike as may be. Wet one till its 



