WARMING AND VENTILATION. 



261 



R, ^ mach 

 f^fw^ jir is re- 

 quired ^r hour 

 by a Lcalthy 

 man'? 



In what man- 

 ner does a heut- 

 «d substance 

 generate a cur. 

 rent of air i 



Fig. 220. 



It is calculated that a fuU-gi'own person of average size ab- 

 sorbs about a cubic foot of oxygeu per liour by respiration, 

 and consequently renders Jive cubic feet of air unlit for breath- 

 ing, since every five cubic feet of air contain one cubic foot of 

 oxygen. It is also calculated that two wax or speroi candles 

 absorb a^ much oxygen as an adult. 



To renaer the air of a room perfectly pure, five cubic feet of fresh air per 

 hour, for each person, and two and a half cubic feet for each candle, should 

 be allowed to pass in, and an equal quantity to pass out. 



588. From every heated substance, an up- 

 ward current of air is continually rising. 



The existence and force of this upward current may bo 



shown in the case of an ordinar3' stove, by attaching to the 



Bide of the pipe a wire on which a piece of thicic paper cut in the form of a 



spiral is suspended, as is represented in Fig. 220. The 



upward current of liot air striking against the surfaces 



of the coil causes it to revolve rapidly around the wire. 



_„ ^ Apart from the consideration of con- 



Why are stoves ^ 



and grates venience, it is necessary that stoves and 



flowf '^^^'^ ^^^ g'"''''^^' intended for warming, should be 

 located as near to the floor of the room 

 as possible ; since the heat of a fire has very little ef- 

 fect upon the air oi" an apartment below the level of 

 the surface upon which it is placed. 



.,-,, , 589. When a fire is lighted in a stove 



Why does ° 



•moke ascend or grate to warm a room, the smoke 

 * ciuniney ? ^j^^^ other gaseous products of combus- 



FiG. 221. tion, being lighter than the air of the room, 



ascend, and soon fill the chimney witii a 

 column of air lighter, bulk for bulk, than 

 a column of atmospheric air. Such a col- 

 umn, therefore, will have a buoyancy 

 proportional to its relative lightness, as 

 compared with the external air and the 

 air of the apartment. ' 



The upward tendency of a column of 

 heated air constitutes the draft of a chim- 

 ney, and tliis draft will be strong and cP 

 fectivc just in the same proportion as th3 

 column of air in the chimney is kept 

 warm. 



Fig. 221 represents a section of a grate 

 and chimney. C D represents tlie light 

 __ and warm column of air within the chim- 

 ney, and A B the cold and heavy column 



