56 RELATIONS OF SOIL TO WATER 



cooled till it becomes supersaturated it will deposit water 

 on the solid bodies with which it may be in contact. If 

 a cold body is introduced into moist air the temperature of 

 the air may be so reduced on the surface of the body that 

 the air becomes supersaturated, and water condenses upon 

 the cold surface. In any case the deposition of dew ie greatly 

 favoured by the presence of dust particles on the solid body. 

 A soil must thus gain water from the air whenever the air is 

 sufficiently moist, and the surface of the soil sufficiently cold to 

 occasion precipitation. It more frequently happens that the 

 deposition of dew or hoar-frost takes place on the herbage 

 occupying the surface rather than on the soil itself. Occasions 

 however do arise in our variable climate in which rather 

 considerable amounts of water are condensed upon the surface 

 of arable soil. These occasions occur most frequently towards 

 the end of winter. The soil is still not far above its minimum 

 temperature when the advancing season brings with it a mild 

 moist wind ; under such circumstances a considerable deposi- 

 tion of water may occur on the surface of the cold soil. We 

 obtain evidence of this fact by studying the results furnished 

 by drain-gauges filled with bare soil. In the case of Mr. Greaves' 

 drain-gauge, containing fine gravel, kept free from weeds, the 

 monthly drainage through the soil has exceeded the monthly 

 rainfall twice in December, seven times in January, seven 

 times in February, and three times in March, during fourteen 

 years. As in each of these months some evaporation must 

 have taken place from the gravel, the amount of water con- 

 densed from the air must have been considerably greater than 

 the differences between rainfall and drainage would indicate. 

 The gravel having little power of retaining water, and being 

 an excellent conductor of heat, was especially suited to exhibit 



