252 



HEATING AS APPLIED IN HORTICULTURE. 



the fire, which reduces the temperature — 

 destroying in one instant all the effect 

 required; liberating the products of the 

 coal in an imperfect state at a low tem- 

 perature—and thus giving rise to volumes 

 of thick smoke, which, when once formed, 

 it becomes impossible to burn with all 

 the heat such a furnace is capable of 

 producing. 



The construction of furnaces, whether 

 for smoke-flues or hot-water boilers, de- 

 serves particular attention ; for on the 

 perfection of their principle much de- 

 pends, as regards both their power of pro- 

 ducing the greatest amount of heat from 

 the smallest quantity of fuel, and of con- 

 tinuing to give out heat for the greatest 

 length of time. One of the greatest faults 

 to be found in hothouse furnaces gene- 

 rally, is their having too great a draught, 

 whereby an unnecessary waste of fuel 

 takes place, and the heat is driven past 

 the boiler, or through the flue and out at 

 the chimney-top, without having time 

 either to heat the water or to communi- 

 cate with the air of the house through 

 the brickwork. Hood's description of a 

 furnace for this purpose is good, and, in 

 fact, contains the essential conditions re- 

 quired. " The heat," he observes, " 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 di- 

 rectly upward to the bottom of the boiler, 

 which will act as a reverberatory, 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 fur- 

 nace with a velocity proportioned 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." 



Amongst the advantages of nearly air- 

 tight furnace-doors, we may state that 

 combustion cannot be supported until 

 the temperature of the air reaches from 

 900° to 1000°— points to which it cannot 

 arrive before it comes in contact with 

 the fuel, by merely passing through the 



chinks of an imperfectly-constructed fur- 

 nace-door. The cause is very different 

 when made to pass through the ash-pit 

 and the bars of the grate, as it becomes 

 heated nearly to that degree before com- 

 ing in contact with the fire. 



According to Tredgold, in a furnace 

 for burning a bushel of coals per hour, 

 the area of the fire-grate should not be 

 less than 8, or more than 16 square feet, 

 and the surface of the boiler should be 

 four times the area of the grate, with 32 

 feet of side flue. This, however, we think 

 considerably more than necessary. A 

 large proportion of grate and bottom sur- 

 face was considered by that eminent en- 

 gineer as greatly tending to lessen the 

 labour of the attendant. 



The following table has been drawn up 

 by Hood for determining the proper area 

 of the bars for furnace grates : " Sup- 

 posing the ordinary kind of furnace-bars 

 afford about 30 inches of opening for the 

 air, in each square foot of surface mea- 

 sured, as the bars are placed in the fur- 

 nace, and allowing half-inch openings 

 between the bars, when the bars them- 

 selves are about 1^ inches wide, then the 

 relative proportions between the area 

 of the bars and the length of the pipe 

 should be as follows : — 



Area of bars. 4-inch 3-inch 2-inch 



pipe. pipe. pipe. 



75 square inches j m f 200 fe t 300 f ^ 

 will supply ) 3 ' 



100 _ _ 200 _ 266 400 ~~ 

 150 _ _ 300 _ 400 „~ 600 ~~ 

 200 _ 400 _ 533 ~~ 800 ~~ 



250 _ _ 500 666 ~~ 1000 _ 



This table is carried to the extent of 

 500 square inches of bars ; but the above 

 may be considered sufficient for our pur- 

 pose, as the proportions upwards are in 

 the same ratio. 



Nicol, Atkinson, and others, used dead- 

 plates both behind and in front of their 

 furnace bars, as already stated ; and some 

 have very judiciously introduced them 

 along the sides also. Dead, or more pro- 

 perly carbonising plates, should be made 

 of strong fire-clay tiles laid round the 

 grating, and with it forming the floor of 

 the furnace. Strong iron plates have also 

 been used, but they are objectionable as 

 being less durable, and as having so great 

 an affinity for oxygen, which all heated 

 metals possess ; — indeed, the less iron em- 



