SCIENCE OF COMMON THINGS. 87 



Haziness of the Indian summer. When vapor forms clouds and fogs. 



higher regions, or upon the point of condensation, which 

 the rising sun cannot disperse. 



Hence our Lord's observation " In the morning ye say, It will be foui 

 weather to-day, for the sky is red and lowering." (Matt. xvi. 3.) 



553 Which is the most transparent, dry or moist air ? 



Air moderately moist is more transparent than very 

 dry air. 



554 What is the cause of the haziness of the atmosphere during that 

 portion of the autumn known as the " Indian Summer f " 



It is undoubtedly due to several causes ; partially to 

 an excessive dryness of the atmosphere, and, in some 

 degree, to the prevalence of smoke in the air arising 

 from burning forests. But it is also a fact, ascertained 

 within a few years, that the constitution of the atmo- 

 sphere is changed in the autumn, and that solar light 

 at that season has less chemical influence than at any 

 other portion of the year. 



555 Why does the sun seen through a fog appear red? 



Because the red rays of light have a greater power to 

 pass through a thick, dense atmosphere than any of the 

 other colored rays. 



556 Why does vapor sometimes form into clouds, and sometimes rest 

 upon the earth as mist or fog? 



This depends on the temperature of the air. "When 

 the surface of the earth is warmer than the lower air, 

 the vapor of the earth (being condensed by the chill 

 air) becomes mist or fog. But when the lower air is 

 warmer than the earth, the vapor rises through the air, 

 and becomes cloud. 



557* Why do clouds often hover around mountain peaks, when the 

 atmosphere elsewhere is clear and free from clouds 1 



It is caused by the wind impelling up the sides of 

 the mountains the warm humid air of the valleys, 

 which in its ascent gradually becomes condensed by 

 the cold, and its excess of moisture becomes visible, 

 and appears as a cloud. 



558 Why are windoios at night often covered with thick mist, and the 

 frames wet with standing water ? 



Because the temperature of the external air always 



