154 THE REASON WHY. 



( Out of the south cometh the whirlwind ; and cold out of the north." 

 JOB xxxvu. 



Whirlwinds are produced by violent and contrary currents meeting 

 and striking upon each other, producing- a circular motion. They 

 generally occur after long calms, attended by much heat. 



Whirlwinds occurring at sea, or over the surface of water, some- 

 times put the water in motion, and as the wind rises upwards it 

 lifts with it a whirling mass of water, producing a water spout. 



Pig. 19. A WATER SPOUT. 



679. Wliy does the chimney smoke when the fire is first 

 lighted? 



Because the air in the chimney is of the same temperature as that 

 in the room, and therefore will not ascend. 



680. Why does the smoking (into the room) cease, after the 

 fire has been lighted a little while ? 



Because the air in the chimney, being warmed by the fire 

 beneath, becomes lighter and ascends rapidly. 



681. Why does a long chimney create a greater draught 

 than a short one ? 



Because the short chimney contains less air than the long one -, 

 there is, consequently, less difference of weight between the warm 



