FIXED STRUCTURES FOR GROWING 



constructed and well-managed flues ; but he allows that in a large pro- 

 portion of the hot-water apparatus now in use the consumption of fuel 

 greatly exceeds that of common furnaces. The cause of the circulation of 

 water in pipes is the same as that which produces the ascent of the air in 

 flues, viz. : difference of specific gravity produced by heat. In water, the 

 particles at the bottom of the boiler being heated, become lighter and rise 

 to the surface, while their place is taken by cold particles from the water 

 in the boiler itself, or in the pipes that communicate with it, which are 

 heated in their turn and ascend to the surface of the water in the boiler 

 and the surface of that in the upper pipe. In like manner, the air heated 

 by the consumption of the fuel in the furnace becomes lighter, and ascends 

 along the flue, while its place among the fuel is supplied by cool air, which 

 enters through the grating beneath it to supply combustion. Neither air 

 nor water will move along readily in very small flues or pipes : for smoke- 

 flues seven inches by ten inches are the smallest dimensions, and hot water 

 does not circulate so rapidly in pipes under two inches in diameter as to 

 give out heat equally throughout their whole length. 



498. The modes of heating by hot water are very numerous, and it would 

 occupy too much room in this work to enter into a detailed description of 

 them, which however is the less necessary as the best modes are sufficiently 

 known for all ordinary purposes by most ironmongers ; and those who wish 

 to make themselves masters of the subject will have recourse to Hood's 

 Practical Treatise on Warming Buildings by Hot Water, published in 1837. 

 The simplest form of applying this mode of heating is by having one 

 boiler to each house in a recess in the back wall, or in some other situation 

 where it will be out of the way, and an upper or flow-pipe proceeding from 

 it on a level with an under or return-pipe, also on a level. Fig. 140 will 



give an idea of this mode 



jjjjj^^ O f circulation, a repre- 



JPP^pFJ^f^m-^^^ senting the boiler, b a 



Fig. 140. A hot-water apparatus for circulation on a level. cistern at the extreme 



end of the house to serve as a reservoir, and e the flue and return-pipes. 

 When the water is to be circulated in pipes on different levels, or above 

 the level of the boiler, or in different levels, but never below the level 



14-2 



Fig. 141. Boiler and furnace for heating by hot water in rising and falling pipes. 

 Fig. 142. Apparatus for circulating water below and above the level of the boiler. 



