8 



which it really is. Such a fog is caused in very nearly the same 

 way as the tiny fog made by breathing. The damp air in the 

 valley has been chilled until the vapor has condensed to form tiny 

 mist or fog particles. Without doubt you can tell why this fog 

 disappears when the sun rises and the warm rays fall upon it! 



On the ocean there are great fogs covering the sea for 

 hundreds of miles, and making sailing dangerous, because the 

 sailors cannot see through the mist, so that two vessels may 

 run together, or a ship may be driven upon the coast before the 



3. — Fog clouds among the valleys in the mountains, only the moutain peaks 



pi'ojecting above them. 



captain knows it. Once more, this is merely condensed vapor 

 caused by chilling air that has become laden with vapor. 

 This chilling is often caused when warm, damp winds blow over 

 the cold parts of the ocean. 



This leads the way to an understanding of a rain storm ; but 

 first we must learn something about the temperature of the air. 

 The air near the ground where we live is commonly warmer 

 than that above the ground where the clouds are. People who 

 have gone up in balloons tell us so ; and now scientific men 

 who are studying this question are in the habit of sending up 

 great kites, carrying thermometers and other instruments, in 

 order to find out about the air far above the ground. 



