THE WALLARAH COLLIERY. 253 



charge that an automatic throttling device has been found 

 desirable at high pressures, in order to avoid shock, strains, 

 and noise. This device consists of a movable bush P, con- 

 trolled by a spring Q; its action is automatic, so that when the 

 trap is discharging at low pressure, as for example when steam 

 is first turned on, the bush does not tend to throttle, but when 

 the pressure rises and there is an increased discharge, the fric- 

 tion thereof moves the bush in an upward direction against the 

 action of the spring, thus reducing the aperture of the dis- 

 charge and consequently its velocity. 



Steam is generated at the E. pit by two Babcock and Wil- 

 cox boilers, and two Cornish boilers, made by Gr. and C. Hos- 

 kins, of Sydney. The Babcock and Wilcox boiler (Fig. 161) 



Fig. 161. Babcock and Wilcox Boiler. 



consists of a number of small steel tubes, which are set at an 

 inclination, the higher point being over the grate. The ends 

 of these tubes are connected by zigzag chambers termed 

 headers. Adjoining headers fit closely together, and are so 

 constructed that the tubes are staggered. The top of each 

 header is connected by a tube to a collecting chamber, one of 

 which is at each end of a horizontal water and steam, drum, ar- 

 ranged overhead. There are openings in the headers opposite 

 each water tube, which are closed by hand holes, the joints of 

 which are made by accurate metallic contact. The tubes are 

 lap welded, so there are no riveted joints, with their double 

 thickness of metal in parts that cause undue strains, owing to 

 unequal expansion. On account of the comparatively small 

 diameter of the tubes, they possess a relatively greater 



