AIR 



as a box where we tumble the bricks in anyhow 

 will hold fewer bricks and so be lighter than the 

 same box if we pack the bricks in close and 

 tightly. We can see this spreading out of warm 

 air by making a very simple experiment. Let 

 us get one of the toy india-rubber balloons that 

 are sold in the street. If we put it too near the 

 fire, it will burst, because the gas inside will 

 expand more than the india-rubber skin is able 

 to do. But if we keep it for a few days, until it 

 gets limp and shabby, because the gas inside is 

 gradually leaking out somewhere, and then warm 

 it, it will fill out again and look quite good as 

 long as it is warm. The cold air is heavier than 

 the warm air, and if they are not mixed by 

 moving objects the warmer air will quickly rise 

 and the colder air will settle underneath it, on 

 the floor in-doors or in hollows outside. 



We all know that warm air goes up the chim- 

 ney and carries the smoke along with it, while 

 we have often felt cold air coming in through the 

 doors and windows to take the place of the air that 

 has gone up the chimney. If we do not leave 

 sufficient room for fresh air to come in easily, it 

 forces its way through cracks and crannies, and 

 we then call these movements of the air draughts. 

 Most people do not like draughts, and since we 

 know now that this entrance of fresh air must 

 take place somehow, we should try to arrange 

 for a constant supply to come into our rooms in 

 a way that will not be unpleasant. This is 



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