156 



VALVES. 



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Fig. 35. shows a section of one of these valves : it is formed of an air-tight 

 cylinder A A, containing a portion of tar or water. B is the inlet-pipe, which 

 communicates with the gasometer ; C is the outlet-pipe, which conveys the 

 gas to the mains ; D D is an inverted cup, ten inches deep, furnished with 

 a rod passing through a stuffing-box, by which it is raised or lowered. When 

 the cup is in the situation shown in the figure, it is evident that the commu- 

 nication between the outlet- and inlet-pipes is shut off by the pressure of a 

 column of water ten inches high. When the cup is raised above the mouth 

 of the outlet-pipe by the rack and pinion, a free passage is left for the gas. 



This description of valve may be fixed with advantage between the gas- 

 holders and the mains, or between any system of lime-water purifiers. Care 

 is necessary that the cup should be sufficiently deep for the required pressure. 



Fig. 36. 



Fig. 36. exhibits a similar valve, differing only in its construction, the outer 

 case serving for a receiver. A is the inlet-pipe ; B the outlet, jointed to the 

 side of the outer cylinder, and communicating to the interior pipe C, which is 

 open at the bottom, its top flanch forming the cover to the outer cylinder. 



