262 WELLS'S NATURAL PHILOSOPHY. 



of air outside the chimney. The column A B being cold and heavy presses 

 down, the column C D being light and warm rushes up, and the greater the 

 difference between the weight of these two columns, the greater will be tho 

 draft. 



A chimney quickens the ascent of hot au- by keeping a long 

 ^°mney°quick- Column of it together. A column of two feet high rises, or is 

 en tlie ascent pressed up, with twice as much force as a column of ono 

 bot^a^r ?'™° " ^oot, and so in proportion for all other lengths — just as two 

 or more corks, strung together and immersed in water, tend 

 ■upward wuth proportionably more force than a single cork. 



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

 lighter than the same bulk of external cold air, if the chimney be one hun- 

 dred feet high, the air or smoke in it is propelled upward with a force of one 

 hundred ounces. 



To what is the ^^ ^lie fire be sufficiently hot, the draft of 

 cWmney°Vo* ^^^ chimnej will be proportional to its length. 



portional ? -poT this reason, the chimneys of large manufacturing estab- 



lishments are generally very high. 



How should a A chimney should be constructed in such 

 constructed? ^ a Way that the flue or passage will gradually 

 contract from the bottom to the top, being widest at the 

 bottom, and the smallest at the top. 



The reason of this will bo evident from the following con- 

 chimney be siderations: — At the base of the chimney, the hot column of 



constructed in expanded air fills the entire passage ; but as the hot air 

 this manner f ^ . , 



ascends it gradually cools and contracts, occupymg less space. 



If, therefore, the chimney were of the same size all the way up, the tendency 



would be, that the cold external air would rush down to fill up the space left 



by the contraction of the hot column of air. This action would still further 



cool the hot air of the chimney and diminish the draft. 



Some persons suppose that a chimney should be made larger at the top than 

 at the bottom, because a column of smoke ascending in the open air, ex- 

 pands or increases in bulk as it goes up. This, however, is owing, in great 

 part, to the action of currents in the air, and to the fact, that a column of 

 smoke freely exposed to the air, is more rapidly cooled than in a chunney, 

 and losing its ascensional power, tends to float out laterally, rather than 

 ascend perpendicularly. 



The causes of " smoky chimneys" are various, 

 rirc^umstal^et* A chimney may smoke for want of a sufficient supply of 



•will a chimney air. If the apartment is very tight, fresh air from without 

 smoke? ^.j^ ^^^ ^^ admitted as fast as it is consumed by the fire, and 



in consequence a current of air rushes down the chimney to supply the defi- 

 ciency, driving the smoke along with it. 



A chimney will often smoke when the heat of the fire is not sufQcient to 



