236 



HEATING AS APPLIED IN HORTICULTURE. 



when the fire, after wasting a prodigious 

 quantity of fuel in the shape of soot, 

 becomes cleared, the heating power which 

 results for a short time is too great for 

 the small quantity of boiler surface 

 exposed to it, and a repellent action 

 ensues between the over-heated iron and 

 the water, similar to that in the well- 

 known experiment of the drop of water 

 in the red-hot capsule. Thus, in a boiler- 

 surrounded furnace, every part of the 

 process is bad. There is, first, imperfect 

 combustion, and consequently a small 

 portion of heat, rendered effectually still 

 smaller by the screen of smoke between 

 the fuel and the boiler; and there is, 

 next, good combustion — too good, indeed, 

 for the distance and quantity of the 

 surface upon which it is to act." 



" It seems rather paradoxical to assert 

 that the boiler can be too close to the 

 fire ; and when I have quoted," he con- 

 tinues " the brassfounders' furnace as an 

 example of good combustion, the reply 

 has been, that the founder closely sur- 

 rounds his crucible with the fuel. And 

 he does rightly, because the melting 

 point of brass is something like 3800°, 

 being fully four times greater than the 

 red heat which is sufficient to excite the 

 repellent action between metal and water, 

 and to stop the farther transference of 

 heat from the one to the other." 



Mr Rogers' conical boiler, which is, in 

 many respects, of a good form, would be, 

 nevertheless, Mr Ainger thinks, " more 



^. AoA effective if set 



Fig. 320. . . 



upon princi- 

 ples more re- 

 sembling that 

 in the annexed 

 fig. 320. The 

 simplicity of 

 the circulation 

 in this and the 

 former case 

 will be no re- 

 commendation 

 to many per- 

 sons ; but I 

 have," he says, 

 " little fear in 

 appealing to 

 experience on this point." 



" With all the care that can be taken, 

 much heat will unavoidably be lost from 

 the exposed surface of the boiler and 



furnace. Where practicable, the boiler 

 should be wholly enclosed with brick- 

 work, built hollow, and the spaces filled 

 with some imperfectly-conducting mate- 

 rial ; or it should be cased in a wrought- 

 iron jacket, and the space between filled 

 with clay or charcoal ; and it is a good 

 practice, where otherwise convenient, to 

 enclose the boiler and furnace within a 

 small chamber, with its fire-door only on 

 the outside.'' — Beknan, in Hist, of Heat- 

 ing, &c. 



In regard to setting boilers, the same 

 excellent authority remarks : " Small 

 boilers are often formed with flues con- 

 ducting the hot smoke round them before 

 it enters the chimney. That portion of 

 boiler surface exposed to the smoke can 

 seldom, in favourable circumstances, be 

 estimated to have an effect equal to one- 

 half of the same extent of fire surface ; 

 in common cases it is much less. Count 

 Rumford found the evaporation nearly as 

 great when the circulation of the smoke 

 in the flue was totally obstructed, as when 

 it was rapid; and side flues he considered, 

 in most cases, useful only in preventing 

 the radiation of heat from the boiler. 

 They are less effective in small than in 

 large boilers. Indeed, flues are seldom 

 introduced into the former; their eco- 

 nomy of heat is doubtful, but their effect 

 certain, in increasing the chances of acci- 

 dents, and hastening the wear of the 

 boiler. The perpendicular smoke-flue, 

 after it leaves the boiler, may have an 

 area double that of the sum of the air- 

 spaces between the bars, and be fitted 

 with a regulating damper. The area for 

 the smoke, when the furnace is in action, 

 will seldom be more than the sum of the 

 air-spaces. Where it can be had, a deep 

 ash-pit is very desirable; but whether 

 deep or shallow, it should in all cases be 

 carefully enclosed by a door, as nearly 

 air-tight as neat workmanship in fitting 

 will make it. The air-valve, which should 

 extend nearly the whole width of the door, 

 or rather the width of the air-spaces, 

 should be formed to have an area, when 

 fully open, equal to the sum of the spaces 

 between the grate bars, and placed as low 

 down, or as near the hearth as possible. 

 The furnace door should be carefully 

 fitted with a similar valved opening to 

 that in the ash-pit door." 



In regard to the water used to fill 



