386 SECTION MECHANICS. 



taken up by the walls of the kiln, is very great : amounting to from 

 two-thirds to three-fourths, in quantity, and frequently to much more 

 in value, as cheap dust coal does the work for which in the older 

 kilns a better coal is required. 



The large amount of water to be evaporated from the unburned 

 bricks can only be carried off, without risk of condensing a portion on 

 the cold bricks next to the chimney, and so wetting and softening them 

 instead of drying them, by drawing through the kiln a very large 

 excess of air, even more than the rude mode of firing adopted renders 

 inevitable ; and as this makes it difficult to maintain a temperature so 

 high as is required for burning fire-bricks, particularly for Welsh 

 Dinas bricks, Mr. Ensor, a fire-brick maker near Burton, adds a by- 

 pass flue to kilns for work of this kind, by which part of the warm air, 

 from the bricks already burned, is led round to the unburned bricks 

 that are being dried, without passing through those compartments in 

 which the highest heat is required ; no more being passed through 

 these than is needed for the combustion of the fuel. In a Hoffmann 

 kiln, thus modified, sufficient heat may be maintained to burn Dinas 

 bricks very satisfactorily. 



Since the continuous Hoffmann kiln has been in use, several forms 

 of what may be termed semi-continuous kilns have been brought 

 forward, in which, as in it, the heat of the burned gases, and of the hot 

 burned bricks, is more or less perfectly utilized. Tunnel-kilns have 

 also been put up, with the same object : in these the position of the fire 

 is fixed, and the bricks or other articles to be burned are drawn past 

 it, stacked on suitable trucks or waggons. 



The small furnaces, fired with coke, that are commonly used for 

 melting steel or brass in crucibles, require no detailed notice. In 

 these, the crucible is imbedded in the fuel, and a rapid combustion 

 and high temperature are maintained round it, by closing the upper 

 part of the furnace and connecting it to a high chimney. 



Where, as in the case of a smithy fire, the top of the furnace cannot 

 be conveniently closed in, or where a keener combustion is required 

 than can be obtained by chimney draught, the plan is adopted of 

 forcing air into the fire by mechanical means. 



Blast furnaces and cupolas, so arranged, are used largely in smelting 



