PLANTS, WITH GLASS ROOFS. 177 



The furnace for a hot-water apparatus has also been subjected to 

 calculation by Mr. Hood. For generating steam, an extremely brisk 

 fire and rapid draught are required; but a very moderate draught 

 will suffice for heating a boiler where the temperature of the water is 

 rarely required to be above 180, or at most 200. The following 

 observations on the construction and management of furnaces are 

 valuable both with respect to a hot-water apparatus and the furnaces 

 to common smoke-flues. a The heat should be confined within the 

 furnace as much as possible, by contracting the farther end of it, at 

 the part called the throat, so as to allow only a small space for the 

 smoke and inflamed gases to pass out. The only entrance for the air 

 should be through the bars of the grate, and the heated gaseous matter 

 will then pass directly upward to the bottom of the boiler, which will 

 act as a reverberator, and cause a more perfect combustion of the 

 fuel than would otherwise take place. The lightness of the heated 

 gaseous matter causes it to ascend the flue, forcing its passage through 

 the throat of the furnace with a velocity proportional to the smallness 

 of the passage, the vertical height of the chimney, and the levity of 

 the gases, arising from their expansion by the heat of the furnace " 

 (p. 77). After giving a table of the area of bars required for pipes of 

 different dimensions and lengths, Mr. Hood observes : " In order to 

 make the fire burn for a long time without attention, the furnace 

 should extend beyond the bars both in length and breadth ; and the 

 coals which are placed on this blank part of the furnace, in conse- 

 quence of receiving no air from below, will burn very slowly, and will 

 only enter into complete combustion when the coal which lies directly 

 on the bars has burnt away." 



The kinds of boilers are very numerous, and few things are 

 more bewildering to the amateur than making a selection from them ; 

 it is nearly equally so to the gardener, as no proper trials have as yet 

 been made to ascertain the best kind of boilers. 



For a small house or a single house of any kind there is no better 

 than the upright oval tubular boiler made by Mr. Gray, Danvers Street, 

 Chelsea, which is the best where there is sufficient depth of stoke-hole. 

 Where this is not the case, a very flat saddle-boiler, made by Mr. 

 Lynch White of Blackfriars, is excellent. For large conservatories 

 and extensive ranges of glass, Weeks's system has unquestionably been 

 proved to be of great excellence, and very recently it has been much 

 improved. Ormson, Green, and many other makers have also good 

 boilers. 



The one-boiler system, by which the whole of the glass of the largest 

 gardens is heated by one fire, is often adopted, and there is doubtless 

 a saving of both labour and fuel by this arrangement. A supplemen- 

 tary boiler should always form part of the one-boiler system, to guard 

 against the tremendous risk of being wholly at the mercy of one boiler 

 for the safety of an entire establishment. The patent duplex improve- 

 ments of Weeks's, however, will probably go far to do away with the 

 necessity for having two boilers. 



Kain- water should, as we have just seen, always be used in hot- 



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