THE PARABOLOIDAL OVEE-GRODND KILN. 207 



whereby the combustible material at the bottom of the chimney 

 may be reached when the kiln is ready to be lighted. The pas- 

 sage is easily made by laying a straight pole on the ground and 

 arranging the billets on each side of it in the way that a house is 

 built up with cards, the pole being finally withdrawn. 



The wood placed immediately against the chimney should con- 

 sist of thin split pieces, dry enough to take fire readily. The best 

 material to use, if obtainable, is the half-charred wood from a pre- 

 viously-burnt kiln. The packing near the chimney should be spe- 

 cially close, all insterstices being filled up with chips and shavings. 



As split wood takes fire most readily on the split side, such wood 

 should be placed with this side facing the chimney or downwards, 

 as the case may be. This position of the pieces also helps the wood 

 to be packed with greater ease and closer together. 



The thickest pieces should be placed where the heat will be strong- 

 est and steadiest, that is to say, about midway between the chim- 

 ney and the periphery. 



4. — Covering the kiln. 



In order to prevent the unchecked entry of air amongst the YtooA. 

 and to regulate the indraught during the carbonization, the cover- 

 ing put over the kiln should be such that, while it is easy to put 

 on and take off or increase and diminish in thickness at any point, 

 it should subside evenly as the kiln subsides, without falling away 

 or opening out in rents and fissures. Experience has shown that 

 it should always consist of two parts, (1) an inner layer composed 

 of moss, sods of turf, green weeds, leafy twigs or green grass, and 

 (2) an outer one of wet -earth plastered or thrown over the first. 



The inner covering must obviously be formed with some green, 

 yielding fibrous material that does not take fire too easily and is 

 at the same time able to hold together, however much the kiln 

 may subside. Moss and close turf are the best for the purpose, 

 and grass the worst. "When grass is used, it ought to be short, 

 soft, and fine. Whatever the material is, it should be the same 

 throughout, otherwise the covering will both lie and subside un- 

 evenly. 



For the outer covering we require a soft earth which will not form 

 a too stiff and impermeable mass when moistened, will not harden 

 and become full of cracks with the great internal heat of the kiln, 

 and will not conduct heat too rapidly, but which will at the same 

 time not lie so loosely as to fall away too easily and not be so porous 

 as to be too freely pei-meablo to air. Hence the best natural 



