SCIENCE OF COMMON THINGS. 



215 



Use of chimneys. 



Utility of long chimneys. 



ney. The column A B being cold and heavy presses down, the column C 

 t) being light and warm rushes up, and the greater the difference between 

 the weight of these two columns, the greater will be the draught. 



14353 IIow do chimneys quicken the ascent of hot air ? 



By keeping a long column of it together. A column 

 of two feet high rises, or is pressed up, with twice as 

 much force as a column of one foot, and so in propor- 

 tion for all other lengths just as two or more corks, 

 strung together and immersed in water, tend upwards 

 with proportionally more force than a single cork. 



In a chimney where one foot in height of the column of hot air is one 

 or.r.ce lighter than the same bulk of external cold air, if the chimney be 

 one undred feet high, the air or smoke in it is propelled upwards with 

 a fore, of one hundred ounces. 



143. To what is the draught of a chimney in all cases proportioned to f 



With sufficient fire, to the length of the chimney. 



1434 Why are the chimneys of large manufactories generally very 

 high ? 



A long chimney causes a current 

 of air to pass through a fire very 

 rapidly, and at the same time very 

 uniformly. On these accounts, for the 

 fires of steam-engines, etc., long chim- 

 neys are preferred. 



1435 When the temperature of the air in a room 

 and of the air outside are the same, will there be any 

 draught up the chimney ? 



There will be no draught. 



143 6 When there is no fire in stove or grate, and 

 the air of a room is warmer than the air outside, will 

 there be a circulation up and down the chimney? 



In such cases there will generally bo 

 two currents, up and down the chim- 

 ney, especially if the doors and win- 

 dows of the room be tight. The warm 

 air of the room will ascend through the 

 chimney, and the cold air descend by 

 the side of it, two currents readily cir- 

 culating through one tube. The direc- 

 tion of the arrows, in Fig. 48, will show Fig. 48. 





