48 Till-: PKlNCll'LKS OF ACKK'rLTL'KE. 



clouds will nut 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, {ind 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 aljove 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 as these pass from the earth, and turn them 

 back, thus preserving an e(|uality 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 rpiantity of water that forms u|)on the ground as 

 dew is nrach larger than is generally supposed. It is 

 only a })i)rtion of it that a])pears in the moi-ning on the 

 blades of.gi-ass. A large part is absoi-bed 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 S(jme 

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

 light rain. 



Frost. — Frost is simjdy frozen moisture from the at- 

 mospliorc. Wlien tlic tempei'ature at the ])()int where 

 (lew would form falls below o2°, the point at which water 



