48 THE PRINCIPLES OF AGRICULTURE. 



clouds will not be formed until a high point is reached, 

 if at all. When it is well saturated, they may be formed 

 so low as to touch the earth, and are then called fogs or 

 mists. 



Dew. Dew, like rain, fog, etc., is caused by the cool- 

 ing of the atmosphere. The earth during the night be- 

 comes cooler than the air above it, and tends to condense 

 moisture from the air which comes in contact with it. 



There is less dew on a cloudy night, because clouds 

 prevent the earth from cooling. They obstruct the rays 

 of heat a.s these pass from the earth, and turn them 

 back, thus preserving an equality of temperature be- 

 tween the earth and the air. 



A strong wind prevents the formation of dew by keep- 

 ing the air well mixed, and leaving no part of it in con- 

 tact with the ground long enough to become cool and 

 deposit moisture. 



A slight breeze increases the amount of dew by 

 removing those portions of air which have already 

 deposited their moisture, and bringing other portions 

 successively in contact with the ground. 



The quantity of water that forms upon the ground as 

 dew is much larger than is generally supposed. It is 

 only a portion of it that appears in the morning on the 

 blades of grass. A large part is absorbed into the soil. 



In Great Britain, where dews are heavy, it is esti- 

 mated that the whole amount deposited in a year would 

 be equal to a depth of several inches of water. In some 

 tropical regions it is deposited so fast as to be equal to a 

 light rain. 



Frost Frost is simply frozen moisture from the at- 

 mosphere. When the temperature at the point where 

 dew would form falls below 32, the point at which water 



