92 SCIENCE OF COMMON THINGS. 



What is snow ? Cause of sleet. 



588 How much moisture can a volume of air at 32 F. absorb f 



An amount equal to the hundred and sixtieth part 

 of its own weight. 



589 How does the capacity of air for moisture increase with the tem- 

 perature ? 



Eor every 27 additional degrees of heat, the quantity 

 of moisture it can absorb at 32 is doubled. Thus a 

 body of air at 32 F. absorbs the 160th part of its own 

 weight; at 59 F. the 80th; at 86 F. the 40th; at 

 113 F. the 20th part of its own moisture. It follows 

 from this that, while the temperature advances in an 

 arithmetical series, the capacity is accelerated in geo- 

 metrical series. , 



590 In what situations is the air always saturated f 



Over the ocean and upon the adjacent coasts. 



591 Where is the absolute humidity of the atmosphere the greatest f 



In the tropics, where the temperature of the air, and 

 its consequent capacity for moisture, is the greatest. 



593 What is snow ? 



The condensed vapor of the air frozen and precipi- 

 tated to the earth. 



593 What is the cause of snow f 



When the air is nearly saturated with vapor, and 

 is acted on by a current of air l>elow the freezing point, 

 some of the vapor is condensed, and frozen into snow. 



A few years ago, some fishermen (who wintered at Nova Zembla), 

 after they had been shut up in a hut for several days, opened the window, 

 and the cold external air rushing in, instantly condensed the air of the 

 hut, and its vapor fell on the floor in a shower of*now. 



594 What is the cause of sleet ? 



"When flakes of snow (in their descent) pass through 

 a bed of air above the freezing point, they partially 

 melt, and fall to the earth as half-melted snow, or 

 sleet. 



595 How does snow prove beneficial to the earth in the cold season f 



It keeps the surface of the earth warm, protects vege- 

 tation to a considerable extent from the cold, and acts 

 as a fertilizer. 



