VALVES. 157 



The cup is brought into action by being lifted and immersing the open end of 

 the pipe C into the tar which it contains. It will be observed, that when it 

 is lowered, the tar contained in the receiver, being higher than the edge of the 

 cup, will run into it and fill up the quantity displaced by C. D is a small 

 service-pipe, through which, by a hand-pump, the superfluous tar may be 

 drawn off. 



This last form of hydraulic valve may be used in the streets, and the cup 

 raised by a screw on the rod, working through a nut at the bottom of the cup, 

 which, being secured from turning round, would follow the thread of the screw. 

 As receivers must be attached at certain distances along the lengths of the 

 main pipe to drain them of water (and a certain portion of tar and oil which 

 still may remain in the gas even after it has travelled miles), they may be 

 easily formed into valves, and thus be made to answer two purposes. A slide- 

 valve is shown in Plate XIX. 



Fig. 1. is an elevation, and Fig. 2. a plan at the top of the stuffing-box. 

 Fig. 3. is a vertical section ; Fig. 4. a plan in section, and Fig. 5. a back eleva- 

 tion of a valve. 



A is a faced cast-iron disc, which by being raised or depressed opens or shuts 

 off the passage for the gas ; it is pressed by the spring B against the part C 

 of the valve-box, which is also faced. 



By the rack and pinion, the disc is drawn up into the position shown 

 by the dotted lines, the spring still causing it to work fair upon the faced 

 part C. 



D is a rod passing through a stuffing-box, and secured by a screw-bolt 

 between the cross feathers cast on the back of the disc A : the same bolt 

 fastens the spring. 



The pillars and cross-head which support the pinion are only used in the 

 works. The arrangement for use in the street-mains is represented by Figs. 

 6 and 7. The rack and pinion is here enclosed within a cast-iron box, the 

 cover being secured by four or five half-inch pins, taped into the sides, and 

 having a hole left for the spindle of the pinion to project through ; by this 

 contrivance no dirt can get to the teeth or to the stuffing-box. The position 

 of the valve is explained by reference to the woodcut, Fig. 37. 



A is the valve laid flat ; B a section of the main ; C a bored wooden block, 

 through which the key is passed when the valve requires to be shut or opened 



