PLANTS, WITH GLASS ROOFS. 



199 



Fig. 138. Section of a greenhouse, with reserve flue and 

 common flue. 



front path ; w, front shelf; n, stage ; and o, path on the upper part of the 

 stage for watering the plants. 



493. The best materials 

 for building flues are bricks 

 and paving tiles, the latter 

 for the bottom and top, and 

 the former for the sides. 

 The advantages of bricks 

 over stone are their greater 

 adhesion to the mortar 

 their narrowness, by which 

 little space is occupied, and 

 their being greater non-con- 

 ductors than stone, by which 

 means the heat is more 

 equalised throughout the 

 length of the flue than it 

 would be by the use of that 

 material. A slight disad- 

 vantage attending the use of bricks and tiles arises from the earth of which 

 they are made ; clay absorbing and entering into chemical combination with 

 the moisture of the atmosphere, especially when the latter is at a high tem- 

 perature. This evil, however, can always be counteracted by placing water 

 over the flues, or in some other hot part of the house. For this purpose, the 

 covers of flues, whether of tiles or stone, ought to be made with sunk panels 

 to contain water ; or, what is much better, a shallow cistern of iron, lead, 

 or zinc, as in fig. 139, may be placed over them for the same purpose. In 

 Germany the flues are sometimes entirely covered with plates of cast-iron ; 

 and if these were formed with turned-up edges, they would serve at 

 once as covers and cisterns. Flues are always detached from the ground, 

 by being built on piers, either connected by low flat arches, or so close 

 together as to be joined by the square tiles which form the floor of the flue. 

 Neither the inside of the flue nor its outside ought to be plastered, when 

 it is desired that they should give out a maximum of heat at a mini- 

 mum of distance from the furnace ; but when the flue is to be of great 

 length, plastering either in the inside or outside, or both, by rendering the 

 walls of the flue greater non-conductors, tends to equalise the heat given 

 out. Plastering is also useful to prevent the escape of smoke from the 

 joints, which is liable to take place where the materials and workmanship 

 are not of the best quality, and to prevent the absorption of moisture by the 

 bricks. Narrow flues are preferable to broad ones, as occupying less hori- 

 zontal space in the house, and also because as flues part with their heat 

 chiefly from their upper surface, it is better equalised by a narrow flue than 

 a broad one. Hence also narrow, deep flues are found to " draw" better 

 than broad, shallow ones. The ordinary dimensions of narrow flues are eight 

 inches in width, and fifteen inches in depth, which is formed by tiles one foot 

 square for the bottom, and ten inches square for the covers, and three paving- 

 bricks, which are only two inches thick, on edge, for each of the sides, as in 

 fig. 139. The joints of the sides and covers are formed by lime putt}", and 

 the bottom tiles are set on bricks on edge. In fig. 139, a is the brick on 

 edge, which supports the one-foot tile ft, which forms the bottom of the flue ; 



