142 COALFIELDS AND COLLIER IKS OF AUSTRALIA. 



knocked about, and it was necessary to get into the solid. The 

 curb bed must be cut perfectly level in rock impervious to 

 water. The Hawkesbury sandstone is treacherous, being full 

 of fissures and false beddings. Should the curb bed not be 

 properly shaped and levelled, endless trouble is caused : the 

 vertical joints are not plumb, and the thrust from the wedges 

 behind do not bring the joints up evenly. It will be noticed 

 from (Fig. 75) that the bottom wedging curb has a rib cast on 

 it 4iin. from the inner end on the top, and a recess on the bot- 

 tom. The latter is unusual, but was allowed in this case for 

 fear it should be necessary to place another length of tubbing 

 immediately below it. The rib on the top of the curb is to 

 prevent the first ring of tubbing from being pushed too far 

 hack. When the curb is laid it is wedged all round the outer 

 side with wooden wedges, care being taken not to push the 

 curb out of alignment. The wedges are driven in until a steel 

 .chisel will not enter to make room for any more. The tubbing 

 proper is made up of 12 segments to a ring: each segment is 

 ^ft. high, and is strengthened by ribs, flanges, and brackets, 

 and has a plug hole in the centre. The segments of each ring 

 break joint vertically, and between all joints wooden sheeting 

 is placed. As there was a pressure of 600ft., the lower half 

 of the tubbing was cast Sin. thick, and the upper half 2|in. 



Fig. 76. Door over Shaft. 



