SUB-SURFACE PACKING 213 



placed five inches apart on an axle. The rim is much 

 thickened and is triangular in shape, with the thin 

 edge outward, so that the effect is to give a decided 

 downward and sidewise pressure, while enough fine 

 earth is left at the immediate surface to serve as a mulch. 

 98. Other surface treatments. Other surface treat- 

 ments aim to decrease the tendency to evaporation. 

 When evaporation takes place into a quiet atmosphere, 

 the layer next to the soil soon becomes so nearly satu- 

 rated with moisture that the rate of evaporation is 

 greatly reduced. But if the atmosphere is in free circu- 

 lation, that is if there is wind, the saturated air is 

 removed, and more dry air is brought over the soil into 

 which evaporation is continuous. The drying effect of 

 wind is very generally recognized. Warm winds in 

 spring and early summer are recognized as particularly 

 drying, and in the semi-arid section just east of the 

 Rocky mountains so-called hot winds sometimes do 

 great damage to growing crops by the rapid evaporation 

 they produce. Obviously anything which reduces the free 

 circulation of the air "breaks the wind" will reduce 

 evaporation. In practice, this takes the form of wind- 

 breaks of various types. Strips of timber are commonly 

 grown or retained for this purpose. Wooden fences 

 and walls of one sort or another have a similar effect. 

 Wind-breaks composed of growing plants have the 

 disadvantage that for a considerable distance beyond 

 the spread of their branches their roots penetrate the 

 soil and use the moisture, which is one reason for the 

 smaller growth of crops near trees. But artificial shelters 

 do not have this advantage. Bearing on the efficiency of 



