116 AGKONOMY 



so because of the facility with which they are able to make 

 new adjustments of their cardinal points. 



Frost. The air always contains some moisture, and the 

 warmer the air the more it can contain. When it contains all 

 that it will hold at a given temperature, it is said to be satu- 

 rated. If the temperature of moist air be lowered beyond the 

 saturation point, some of the moisture has to be dropped. If 

 this occurs in the air, the dropped moisture is called rain or 

 fog ; if deposited on the earth, it is dew or frost. The point 

 at which the water begins to condense out of the air is called 

 the dew point. Frost differs from dew only in that it is depos- 

 ited at temperatures below the freezing point of water. There 

 is always danger of frost when the weather report predicts 

 temperatures eight or ten degrees above freezing, since in any 

 given locality the temperature may fall a few degrees below 

 that predicted. A still clear night favors the formation of 

 frost by promoting the cooling of the earth and air by radia- 

 tion. When the sky is cloudy, the clouds, like a blanket, keep 

 in the heat. A fog at night, or much smoke or dust in the air, 

 protects in the same way. A windy night may also protect 

 from frost by moving the cold air about and keeping it from 

 settling down in any one place. 



Locality and frost. Danger from frost is not confined to 

 northern latitudes. In any region where the temperature goes 

 below the freezing point there is danger from late spring and 

 early autumn frosts. The location, however, often has much 

 to do with immunity from frost. In the vicinity of large bodies 

 of water frosts are often long delayed in autumn because, as 

 the temperature lowers, the water gives off the heat absorbed 

 during the summer and thus keeps the surrounding air warm. 

 In rooms or cellars where the temperature might fall below 

 the freezing point, this may be prevented by exposing therein 

 tubs of water which will give off heat in the same way. Cold 

 air is heavier than warm air and tends to settle in the hollows 



