BOOK III. 



STEAM BOILERS AND TUBES. 



327 



too wide, or do not ascend from the furnace to the chimney ; steam is impelled from the 

 boiler and will proceed with equal rapidity along small tubes or large ones, and descend- 

 ing or ascending. Finally, with steam, insects may be effectually kept under in hot-houses, 

 with the greatest ease, by merely keeping the atmosphere of the house charged with vapor 

 from the tubes for several hours at a time. 



1666. The disadvantages of steam as a vehicle for conveying heat to hot-houses are few. 

 On a small scale it is more expensive than the mode by flues, and more trouble is required to 

 attend to one boiler than to one or even two or three furnaces. These are all the dis- 

 advantages we know of. It has been stated by some that steam draws up or etiolates bo- 

 tanic plants, and lessens the flavor of fruits ; but we are inclined to consider such effects, 

 when attendant on plants or fruits in houses heated by steam, as resulting from some de- 

 ficiency of management in other points of culture. 



1667. The boilers used to generate steam are formed of cast or wrought iron, or copper, 

 and of different shapes. Wrought-iron and an oblong form are generally preferred at 

 present, and the smoke-consuming furnace most approved is that of Parkes. 



1668. The tubes used for conveying steam are formed of the same metals as the boilers ; 

 but cast-iron is now generally used. Earthen or stone ware tubes have been tried ; but it is 

 extremely difficult to prevent the steam from escaping at their junctions. The tubes are laid 

 along or around the house or chamber to be heated, much in the same manner as flues, only 

 less importance is attached to having the first course from the boiler towards the coldest 

 parts of the house, because the steam-tube is equally heated throughout all its length. As 

 steam circulates with greater rapidity, and conveys more heat in proportion to its bulk, 

 than smoke or heated air, steam-pipes are consequently of much less capacity than smoke- 

 flues, and generally from three to six inches diameter inside measure. Where extensive 

 ranges are to be heated by steam, the pipes consist of two sorts, mains or leaders for sup- 

 ply, and common tubes for consumption or condensation. Contrary to what holds in 

 circulating water or air, the mains may be of much less diameter than the consumption 

 pipes, for the motion of the steam is as the pressure ; and as the greater the motion, the 

 less the condensation, a pipe of one inch bore makes a better main than one of any larger 

 dimension. This is an important point in regard to appearance as well as economy. 

 In order to procure a large mass of heated matter, M'Phail and others have proposed to 

 place them in flues, where such exist. They might also be laid in cellular flues built as 



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