SCIENCE OF COMMON THINGS; 75" 



Air, how heated and cooled. Origin of winds. 



463 In what manner is the air heated by the earth ? 



The sun heats the earth, and the earth heats the air 

 resting upon it ; the air thus heated rises, and is suc- 

 ceeded by other air, which is heated in a similar way, 

 till the whole volume is warmed, 



464 How is the air made cold? 



The air resting on the earth is made cold by contact / 

 this cold air makes the air above it cold / and cold cur- 

 rents (or winds) cause the whole to mix together, until 

 all becomes of one temperature. 



465 What effect is produced upon air "by cold f 



It is condensed or compacted into a smaller compass ; 

 in consequence of which it becomes heavier, and de- 

 scends towards the ground. 



466 Prove that the air is condensed ly cold. 



Lay a bladder half full of air before a fire, till it has 

 become inflated ; if it be now removed from the fire$ 

 the bladder will collapse again, because the air con- 

 denses into its former bulk. 



467 What effects has heat upon air f 



Heat rarefies or makes it lighter ; that is, a quantity 

 of air heated will occupy more space than the same 

 quantity which has been cooled. 



468 What is wind f 



"Wind is air put in motion. 



460 What occasions- those movements of the air which we call wind? 



The principal cause is the variation of temperature 

 produced by the alternation of day and night and the 

 succession of the seasons. 



4"7O How can winds originate through variations of temperature f { 



"When through the agency of the sun a particular 

 portion of the earth's surface is heated to a greater 

 degree than the remainder, the air resting upon it 

 becomes rarefied and ascends, while a current of cold 

 air rushes in to supply the vacancy. Two currents, the 

 one of warm air flowing out, and the other of cold air 

 flowing in, are thus continually produced ; and to these 



