218 THE PRISMATIC CHAROOAL KILN. 



To secure a through draught, a tunnel (it) is left along the 

 whole length of the kiln, and it is by this tunnel, -which is after- 

 wards filled -with small combustible wood, that th-e kiln is fired. 

 At V the covering is omitted, in order to draw the fire inwards and 

 upwards. The fore end of the tunnel is left more or less open 

 fcr several hours after combustion has begun. On its being closed, 

 several holes are made both in the outer face and sides of the kiln 

 end also one on each side of the vent-hole v, which itself is kept 

 open until carbonization is nearly complete. 



The outer face and sides of the kiln are often rather steep, and 

 th-e covering of earth must then be kept in place with the aid of 

 struts, as explained in Mcf. 67 above. 



In a report written by Mr. Heinig in 1880, whilst he was at the 

 Forest School, he says that be found it an advantage to form a 

 vertical chimney rising up from the inner extremity of the tunnel, 

 and to fire the kiln through this chimney as well as through the 

 tunnel. The chimney, no doubt, renders the firing very ranch 

 easier, bat its absolute utility has still to be proved by a larger 

 number of experiments than he was able to try. The best way to 

 secure the carbonization of the wood near the hill is to make the 

 height of the kiln diminish towards the hill and to keep the vent- 

 holes open on the top along the edge of the cutting. 



With skilful burning, the yield in this style of kiln should hard!v 

 if at all, fall short of that obtainable from any other kind of kiln. 



Aeticle 4.— The Peismatic Kilw. 



The shape of the prismatic kiln resembles to a certain extent 

 an ellipsoidal dome springing up from a rectangular base. If I and 

 b be respectively the length and breadth of its base, and h its dimen- 

 sion where the height is greatest, then the stacked contents will 

 be approximately = J Z & A. 



Such kilns are most conveniently built up with straight long 

 pieces, of more or less the same thickness, running through the 

 entire length of the kiln ; but the prismatic shape is very frequent- 

 ly adopted even in the absence of such pieces, because it is on the 

 whole easier to form and requires the wood to be much less cut up 

 than the paraboloidal form, although it is, on the other hand, more 

 liable to breakages from irregular settling and more difficult to 

 cover properly. There is no chimney, but a tunnel is left along 

 the ground running through the entire length of the kiln and filled 

 with combustible material, which is fired at both ends. In India 

 prismatic kilns are usually made much larger than paraboloidal 

 ones. The outturn of charcoal does not difi'er materially from that 



