HEATING BY FLUES. 



147 



which, in building, makes them as strong 

 as ordinary flues. The bricks and covers 

 are hollowed out in the middle to half 

 their thickness, by which means the heat 

 is sooner transmitted through them, and 

 in consequence heats the house sooner 

 than if the bricks were of the usual thick- 

 ness, thereby answering the purpose for 

 which they were intended. 



Flag-stone flues. — Where stone of a pro- 

 per quality for standing heat is to be had, 

 very neat flues may be constructed of it. 

 The thickness of the stone should not be 

 less than 2 inches, in lengths as great as 

 can be got, to lessen the number of joints. 

 The joints should be all half-checked and 

 finished with fire-clay. The sides should 

 be secured by being batted with iron 

 bats, placed inside, and flush with the 

 surface of the stone. Caithness pave- 

 ment is well adapted for this purpose, as 

 it stands the heat well, and can be had of 

 any reasonable length, say 10 feet, and 

 of any thickness ; and if rubbed or half- 

 polished, it would have a very elegant 

 effect. 



Cast-iron flues. — These have been re- 

 commended as being very durable ; but 

 the action of the heat on metallic sub- 

 stances is such as to render their adop- 

 tion anything but advisable. Sir George 

 Mackenzie has recommend triangular flues 

 of this material, covering their surface 

 with a mixture of clay and sand; this, 

 however, would only lessen their dele- 

 terious effects to a very limited extent. 



Common or detached flue — fig. 151. — This 

 was the first real improvement in flue- 



Fig. 151. 



building. Flues of this description vary 

 in dimensions from 9 to 12 inches in 

 width, and from 12 to 18 inches in height. 

 They are built of regular and well-sized 

 bricks placed on edge ; but where a great 

 and constant heat is required, they should 

 be on bed, being thus much stronger, and, 

 when once heated, retaining their heat 

 much longer. They ought to be neatly 

 jointed with well-prepared ground mortar, 



and washed inside with thick grouting, 

 but plastered neither inside nor out. The 

 foundations being formed as already re- 

 commended, and brought to a proper 

 level, the bottom or floor of the flue should 

 be formed of pavement or tiles, as may be 

 most convenient, and elevated upon brick 

 piers, so as to raise it about 6 inches 

 above the finished floor of the house. 

 This is for the purpose of keeping the 

 flue free from damp, which would have 

 a tendency to cool the air in it, and to 

 obstruct the draught or current of smoke 

 and heat, which are both lighter than 

 cold and particularly damp air, as well 

 as of preventing the loss of heat by ab- 

 sorption. The best covering is pavement 

 that will stand the heat, or tiles either of 

 brick earth, or fire-clay. For greenhouses, 

 where a moist heat is not required, they 

 may be level on the top ; but for forcing- 

 houses and plant-stoves, they may be 

 hollowed out in the middle to hold water, 

 for the purpose of raising steam or vapour, 

 by pouring water upon them. Although 

 we recommend this provision to be made 

 for steaming, we do not by any means 

 approve of the manner in which this 

 operation is too frequently performed — 

 namely, by pouring unnecessary quanti- 

 ties of water on them. Little at a time 

 should be put on, and this frequently 

 repeated. When the tiles are porous, 

 much of this water filters through into 

 the interior of the flue, chilling the tem- 

 perature in it, and checking the draught. 

 It is a good plan to have such covers 

 glazed by the potter, or painted with oil 

 colour, to prevent infiltration. But the 

 most uniform of all evaporations is ob- 

 tained by setting shallow earthenware 

 or copper vessels along the top of the 

 flue, and keeping them full of water. The 

 coverings of flues seldom project over the 

 face of the bricks, particularly where tiles 

 are used. When pavement is used, it 

 sometimes projects 1^ or 2 inches by way 

 of protection to the flue ; but this projec- 

 tion, to a certain extent, prevents the 

 direct ascent of the heat. Often, when 

 tiles are used, they are narrower than the 

 external breadth of the flue, the defi- 

 ciency being made good by chamfering the 

 space off with mortar. In laying on the 

 covers, whether stone or tiles, it will be 

 well to place under every joint a slip of 

 hoop-iron 3 inches wide; this is not 



