24 TILE DRAINAGE. 



So it will for a time and at the top, until it dries the top half 

 an Inch or so and makes it a dry mulch between the hot air 

 and wind and the top ends of the capillaries. For heat and 

 wind, as we have already seen are the other agents besides 

 f lost that can take the moisture as fast as the capillary pores 

 can lift it to the surface. But the heat and wind must be 

 able to come in contact with the upper ends of the capillaries 

 in order to take their moisture ; and this the dry-earth 

 mulch almost wholly prevents ; for dry earth is a very poor 

 conductor, both of warmth and air and of capillary water. 

 The fine tillage of the surface destroys the fine small capilla 

 ry pores at their top ends, and substitutes coai ser but still 

 very small air spaces in the drying earth, which will not 

 readily conduct the water from the capillaries to the top of 

 the ground, nor admit currents of warm air to reach the top 

 ends of the capillaries. For, to be more exact, it is not the 

 heat or the wind that takes up the moisture pumped to the 

 surface by the capillaries, but the air (or atmosphere) itself. 

 This has the physical property or power of absorbing and 

 holding water in large quantities in the form of vapor ; and 

 the warmer the air is, the more w r ater (vapor) it will hold. 

 Heat turns the water into vapor, and increases the power of 

 the air to absorb it. Wind aids in absorbing the vapor from 

 the ground, for wind is simply moving air ; and when the 

 wind blows, new portions of air pass along and come in con- 

 tact with the damp surface of the ground and take up or 

 absorb what vapor they can hold ; and then they move along 

 and give other drier portions a chance to load up with water 

 like the line of empty buckets at a fire, filling up at the 

 well or tank or hydrant, and passing on to give place to 

 other empty ones. That is the reason why a windy day is 

 usually a drying one. And so while deep coarse tillage helps 

 to dry out the soil, fine surface tillage after showers does 

 really help retain moisture. The capillaries cease to pump 

 it up, because the wind and heat are Ifcept by this mulch 

 from drinking it up ; just as the capillaries of a lamp- wick 



