30 CHANGES IN THE ATMOSPHERE. 



atmosphere, and is shown by the difference between the 

 two thermometers. 



93. By means of these three instruments, and knowing 

 how to use them, an intelligent husbandman may select 

 the moment most favorable or most important for certain 

 operations ; and can often predict, with an approach to 

 probability, what changes will take place in the weather 

 before night or before the next morning. 



94. The Variations in the Temperature of the air depend 

 first, upon the seasons, from the cold of winter to the 

 heat of summer ; 2d, upon the direction of the wind, 

 some winds always bringing cold, others always bringing 

 heat; 3d, upon* the clouds, which prevent the sun s light 

 and heat from falling upon the earth. 



95. The atmosphere being in cbntinual motion, like 

 the waters of the ocean, the column of air over us is 

 sometimes longer and heavier, and sometimes shorter. 



96. Variation in the Moisture of the air depends chiefly 

 upon the winds, which bring on air more or less abun 

 dantly charged with moisture, according as they have 

 passed over seas, lakes, or rivers, or over a continent. 

 In the Atlantic States gf America, the easterly and 

 southerly winds, coming from over the ocean, are always 

 full of moisture. The south and west winds, coming 

 from warmer regions, arc warm, and, in proportion as 

 they are more westerly, are dryer winds. The north and 

 west winds, coming from the mountains and plains of 

 the continent, arc dry and cold. The coldest and dryest 

 are the north wind and the north-west wind, and any 

 wind from a point between the two. 



The moisture also depends on the temperature. Heat 

 dissolves moisture as water dissolves salt. When the air 

 is warm, it can contain a great deal of moisture ; but as 



