PLANTS, WITH GLASS ROOFS. 



167 



the floor of the house; A, the reservoir flue, three feet broad, which is 

 sunk so that its top is on a level with the floor ; e', an air-drain from the 



back of the house ; &, an 

 upper flue for additional 

 heat; /, front path; TO, 

 front shelf; , stage ; and 

 o, path on the upper part 

 of the stage for watering 

 the plants. 



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 narrow- 

 ness, by which little space 

 is occupied ; and their 



Fig. 138. 



Section of a greenhouse, with reserve Jlue and 

 common flue. 



being better non-conductors than stone, by which means the heat is 

 more equalized throughout the length of the flue than it would be by the 

 use of that material. Stones are likewise very liable to split and burst 

 into fragments when overheated. A slight disadvantage 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 mois- 

 ture of the atmosphere, especially when the latter is at a high tempe- 

 rature. 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 con- 

 nected 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 minimum 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 equalize 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 

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

 heat chiefly from their upper surface, it is better equalized by a 



