THE SYDNEY HARBOUR COLLIERY. 135 



drawing the keys of the internal drum, and turning it by 

 hand. The section of a Cornish double-beat valve, with the 

 valve raised, is shown in Fig. 70 : the lower seat is the ring 

 (A), the upper, the plate (B), supported above (A) by the 

 wings (C), and bolted to the bridge piece (D). As 

 (B) exactly covers the opening, though at a higher 

 level, the valve when closed is entirely shielded from 

 the steam pressure below, so far as that power tends to lift or 

 depress the valve; the latter is, therefore, only the recipient 

 of horizontal pressure, consequently the valve is wholly bal- 

 anced ; in other words, the rod (E) has merely to lift the dead 

 weight. Both the exhaust and steam valves are worked by 

 means of rockers. 



The engine foundation is of brick, set in lime mortar, 

 and the whole is housed in a structure built of timber 

 and galvanised iron. 



The Jubilee shaft engine is the same type as the Birthday 

 engine, only the cylinders are 30in. in diameter, and the wind- 

 ing drum 13ft. diameter, by 5ft. liii. wide. This will be part 

 of the permanent plant, at least until the output is so great 

 that both shafts will have to be used for hoisting purposes at 

 the same time. This engine is bolted down to solid rock. 

 The engine house is of brickwork set in cement mortar, and it- 



Fig. 71. Headf ranies, Sydney Harbour Colliery. 



