170 COALFIELDS AND COLLIERIES OF AUSTRALIA. 



At first, when the seam was thin, from 2ft. llin. to 3ft. 

 6in., the coal was extracted on the long-wall system; but where 

 the seam increases to 4ft. Gin. and over, a change is made 

 to pillar and bord, as there is not sufficient stone to 

 support the roof when required. The face of the longwall is 

 semi-circular in shape, and stall-roads leading to it are 

 10 yards apart from centre to centre ; they are kept open by 

 pack-walls built up of stone lifted from the bottom (which is 

 shale, and easier to work than the sandstone roof) in order 

 to obtain head room. The longwall method depends largely 

 for its success on the travelling weight of the roof, which rolls 

 forward and bears down on the strip of undercut coal at the 

 face, causing it to fall. The rock overhead may be divided 

 into two parts according to its action: 1st. the underweight 

 or fractured material (A) Fig. 104. below the arch, and the 



Fig. 104 Weight of Hoof. 



overweight or solid strata (B) above the arch, which settles 

 on the packs behind, and keeps pace with the travelling or 

 underweight. The roof at North Bulli fractures 15ft. to 

 16ft. behind the face, and the overweight crushes the packs 

 to about half their original size. Though roof pressure 

 cannot be overcome, it can be regulated by constant work, 

 and a uniform line of face, so that it is just sufficient to 

 break down the strip of undercut coal. If too great, the roof 

 pressure will crush the coal, and may cause the roof to 

 break off short at the face. The systematic timbering of a 

 longwall face is of great importance. The props should be 

 set in rows parallel to the general line of the working face ; 

 never staggered. The regular drawing of the back timber 

 next the packs causes the underweight to settle forward on 

 the face of the coal. The settlement of the overweight is 

 regulated by the proportion of pack walls, and the manner of 

 building them up. If the overweight settles too rapidly it 



