SCIENCE OF COMMON THINGS. 213 



Causo of air currents at windows and doors. What is a vacuum ? 



This is not the case if a fire be in the room. When a fire is lighted, 

 an inward current is drawn through all the crevices. 



1430 Why would the flame 'be blown outvjards (towards the hall) if a 

 candle be held at the top of the door ? 



Because the air of the room being heated, and con- 

 sequently ratified, ascends, and (floating about the 

 upper part of the room) some of it escapes through the 

 crevice at the top of the door, producing a current of 

 air outwards (into the hall). 



1431 Why would the flame be blown inwards (into the room) if the 

 candle be held at the bottom of the door ? 



Because a partial vacuum is made at the bottom of 

 the room, as soon as the warm air of the room has 

 ascended to the ceiling or made its escape from the 

 room ; and the cold air from the hall ruslies under the 

 door to supply the void. 



1433 What is meant by a partial vacuum being made at the bottom 

 of the room ? 



It here signifies a place from which the air has been 

 taken / and a " partial vacuum" means a place from 

 which a part of the air has been taken away. Thus, 

 when the air near the floor ascends to the ceiling, a 

 partial vacuum is made near the floor. 



1433 And how is the vacuum filled up again f 



It is filled up by colder air, which rushes (under the 

 door, and through the window crevices) into the room. 



1434 Give me an illustration. 



If I dip a pail into a pond and fill it with water, a 

 hole (or vacuum) is made in the pond as big as the 

 pail ; but the moment I draw the pail out, the hole is 

 Jilled up by the water around. 



1435 Show how this illustration applies. 



The heated air which ascends from the bottom of a 

 room is as much taken away as the water in the pail, 

 and (as the void was instantly supplied by other water 

 in the pond) so the void of air is supplied by the air 

 around. 



1436 Why is a room (even without a fire) generally warmer than ifa 



epsn air? 



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