HEATING BY FLUES. 



149 



of a boiler at all. Fig. 155 is a section 



Fig. 155. 



through the flue and tank; and fig. 156 

 is a cross section through 

 the boiler and furnace, 

 and end flue of the house, 

 showing the ventilation 

 in front at a, the air en- 

 tering by it, ascending 

 into the house through the 

 space between the front 

 wall and tank, and also through square 

 flues passing under the tank, and through 



Fig. 156. 



the bridges that divide the large flue into 

 chambers. These small air-flues are 

 shown by the shaded squares in the longi- 

 tudinal section ; b is the leaden boiler ; 

 c the register for admitting cold air from 

 without to pass over the boiler, and so on 

 into the flue ; d the furnace ; e the ash-pit ; 

 / the leaden flow-pipe carrying the water 

 from the boiler to the tank ; the return- 

 pipe is close to it, but cannot be shown. 

 The arrows show the direction of the 

 smoke. In consequence of this house 

 being converted into an aquarium, these 

 flues, after existing for six years, have 

 been removed. 



Amongst recent improvements in flue- 

 building, we may notice that our intelli- 

 gent friend, Mr Lyle, surgeon, Newburgh, 

 has lately erected a flue of greater calibre 

 than usual, and arched over instead of 

 being covered with tiles. This flue has 

 also a rather singular feature — namely, 

 it increases in size as it recedes from the 

 fire, which is said to economise fuel and 

 maintain a uniform heat. It has been 

 stated as an objection to this theory, that as 

 the air becomes expanded its capacity for 

 heat is multiplied, and, consequently, less 

 heat would be evolved from it. This is, 

 however, asserted by Mr Lyle not to be 

 the case : " as to the property of expanded 



air possessing a greater capacity for heat 

 than air more dense, this is perfectly cor- 

 rect; but I wish," he says, in the " Gar- 

 deners' Chronicle," " to explain a fallacy 

 that has been fallen into on the applica- 

 tion of that principle. The rule for air, 

 as well as all other gases, is, that its capa- 

 city for heat enlarges in nearly an inverse 

 ratio to its density — that is, at half the 

 density it has nearly double the capacity; 

 or, in other words, in the former that it 

 will take nearly double the quantity of 

 heat that it would in the latter, to raise it 

 to the same degree of temperature : this 

 is the cause of difference of climate at 

 different altitudes under the same parallel 

 of latitude," A mistake has been fallen 

 into by imagining that " because hot air 

 moves along a flue gradually enlarging 

 from the fire end, that this air must gra- 

 dually expand also in the same propor- 

 tion, and must have its capacity for heat 

 increased, absorb it, and that it does not 

 give it off to the walls of the flue. But 

 the very reverse of this is the fact — the 

 air in the flue, instead of expanding and 

 becoming more rarefied in its course, 

 contracts, or becomes denser; and it 

 does this because it gets colder in its pas- 

 sage along the flue. Air doubles its bulk 

 for every 480° of increased temperature ; 

 and hence, in its progress along the flue, 

 for every degree of heat it loses, it con- 

 tracts ^ 5 in volume, or gets by that 

 fraction more dense; and, of course, in 

 proportion, diminishes its capacity for 

 heat, giving it off in a sensible state to 

 the brickwork around. The plain effects 

 of a brick flue gradually enlarging from 

 the fire are these: — 1st, A current of hot 

 air diminishes in velocity as it recedes 

 from the fire, so that, where the air is 

 hottest, it has least time to give off its 

 heat, and where coldest, most time for 

 this purpose. 2d, The radiating surface 

 the temperature gets 

 lowest, and com- 

 pensates as much 

 as possible for 

 that deficiency." 



Circular flues 

 have been recom- 

 mended by Mr 

 Walsh, the bricks 

 for which are 

 made for the ex- 

 press purpose ; 



increases where 

 Fiff. 157. 



