128 EFFECTS OF HEAT. 



Q. What is the difference between a FOG and 

 a CLOUD ? 



A. Clouds and fogs differ only in one 

 respect. Clouds are elevated above our 

 heads : but fogs come in contact with the 

 surface of the earth. 



Q. If CLOUDS are WATER, why do they FLOAT 

 on the air ? 



A. 1st The vapour of clouds is 

 composed of very minute bubbles (called 

 ves'cicles), which float like soap bubbles : 

 and 



2ndly Warm air (between the 

 bubbles) keeps them apart, and makes the 

 mass lighter; and the currents of air 

 (which constantly ascend from the warm 

 earth) buoy them up. 



Q. Why does VAPOUR sometimes form into 

 CLOUDS, and sometimes rest upon the earth as MIST 

 or FOG ? 



A. When the surface of the earth is 

 warmer than the air, the vapour of the 

 earth (being condensed by the chill air) 

 becomes mist or fog. But when the air 

 is warmer than the earth, the vapour rises 

 through the air, and becomes cloud. 



