-WARMING AND VENTILATION. 2G3 



rarefy all the air in the chimney ; in such cases the cold air (condensed in tho 

 upper part of the flue) will sink from its own weight, and sweep the ascend- 

 ing smoke back into the room. 



"When the fire is first hghted, and the chimney is filled with cold air, there 

 is often no draft, and consequently the flame and smoke issue into tho room. 

 This, in most cases, is remedied by the action of a " blower." 



A blower is a sheet of iron that stops up the space above 

 ^of a blower? *^® grate bars, and prevents any air from entering the chim- 

 ney except that which passes through the fuel and produces 

 «ombustion. This soon causes the column of air in the chimney to become 

 heated, and a draft of considerable force is speedily produced through the 

 fire. The increase of draft increases the intensity of the fire. 



Another frequent cause of smoky chimneys is, that when the tops are 

 commanded by higher buildings, or by a hill, the wind m blowing over them, 

 falls hke water over a dam, and beats down the smoke. The remedy in such 

 cases is, either to increase the height of the chimney, or to fix a bonnet or 

 cowl upon the top. The pMlosophy of this last contrivance consists in the fact 

 that in whatever direction the wind blows, tho mouth of the chimney is 

 averted from it. 



In a room artificially heated, there are al- 



What is the , , /• • j? i . • n 



motion of the wajs two curreuts 01 air ; one oi not air now- 

 artificiaiiyheat- insc out of the room, and another of cold air 



ed* 



flowing into the room. 



If a candle be held in the doorway of such an apartment, near the floor, it 



will be found that the flame will be blown inward ; but il' it be raised nearly 



to the top of the doorway, the flame will be blowm outward. The warm air, 



in this case, flows out at the top, while the cold air flows in at the bottom. 



590. An open fire-place diflers greatly from a close stove 

 How does a ., . . , ,, /■ j 



stove diflfer m respect to ventflation, uiasmuch as the lormer warms ana 



from an open ventilates an apartment, whUe the latter only warms, and can 

 fire-place in ' ,, i -i • t 



respect to ven- hardly be said to contribute at all to the ventilation, in a 

 tUation? gjjjgg gtQvg^ no air passes through the room to the flue of 



the chimney, except that which passes through the fuel, and the quantity 

 of this is necessarily limited by the rate of combustion maintained in the 

 stove. In an open fire-place, a large amount of air is continually rushing up 

 the chimney through the opening over the grate, irrespective of what passes 

 through the fire and maintains combustion. 



In summer time, when no fire is made in the chimney, the column of air 

 in it is generally at a higher temperature than the external air, and a current 

 will therefore ip such case be established up the chimney, so that the fire- 

 place will still serve, even in the absence of fire, the purposes of ventilation. 

 In very warm weather, however, when the external air is at a higher tem- 

 perature than the air within the building, the eff^jcts are reversed ; and the 

 air in the chimney being cooled, and therefore heavier than the external air, a 

 downward current is established, which produces in the room the odor of soot 



