SCIENCE OF COMMON THINGS. 147 



What renders clothing necessary ? Use of woollens and furs. 



In the case of a stove or furnace, the air which is in 

 contact with the tire or the heated surface, first becomes 

 heated, expands, and rises ; cold air rushes in to supply 

 its place, is heated, and ascends in like manner, and 

 this interchange goes on until all the air in the room is 

 heated. 



946 If air be a bad conductor of heat, why should we not feel as warm 

 without clothing as when we are wrapped in wool and fur f 



Because the air (which is cooler than our body) is 

 never at rest j and every fresh particle of air draws off 

 a fresh portion of heat. 



947* How does the ceaseless change of air tend to decrease the warmth 

 of that part of the body devoid of clothing ? 



Thus : the air (which cases the body) absorbs as 

 much heat as it can while it remains in contact ; being 

 then blown away, it makes room for a fresh coat of 

 air, which absorbs more heat. 



948 Does the air which encases a body devoid of clothing become (by 

 contact) as warm as the body itself? 



It would do so if it remained motionless ; but as it 

 remains only a very short time, it absorbs as much heat 

 as it can in the time, and passes on. 



949 Why do we feel colder in windy weather than in a calm day f 



Because the particles of air pass over us more rapidly, 

 und every fresh particle takes from us some portion of 

 heat. 



950 Why are woollens and furs used for clothing in cold weather? 



Because they are very bad conductors of heat ; and 

 therefore prevent the warmth of the body from being 

 drawn off by the cold air. 



951 Do not woollens and furs actually impart heat to the body f 



No ; they merely prevent the heat of the body from 

 escaping. 



953 Where would the heat escape to, if the body icere not wrapped in 

 wool or fur 1 



The heat of the body would fly off into the air ; for 

 the cold air (coming in contact with our body) would 



