1070 



VENTILATION OF MINES 



pressure of about 13 Ibs. per square inch. The excentric L for the slide valve is 

 placed just inside the air-trunk E, and works the valve through a short-weight shaft M, 

 vdth a lever on the outside. 



The pit H, fig. 2085, is of an oval form, 10 feet by 18 feet, and divided near the 

 centre by a timber brattice N, the one side forming the upcast shaft and the other the 

 downcast. Both of these are used for winding, and the cages o, in which the trucks, 



2086 



2087 



&c., are brought up, Vork between guides fixed to the timbering of the pit. The 

 pumps P are placed in the downcast shaft. 



In order to allow of the upcast shaft being used for winding, the top is closed by an 

 air-valve B, which is formed by simply boarding up the underside of the ordinary 

 guard upon the mouth of the shaft, leaving only the hole in the centre through which 

 the chain works. This air-valva B is carried up by the cage o on arriving at 



the top of the shaft, as in fig, 

 2085, and then drops down again 

 flat upon the opening when the 

 cage is again lowered. During 

 the time that the valve is lifted, 

 its place is occupied by the close 

 bottom of the cage o, which nearly 

 fills the rectangular opening left 

 at the top of the shaft. By this 

 simple means it is found practi- 

 cally that a complete provision is 

 made for keeping the top of the 

 upcast shaft closed, and main- 

 taining a uniform current of air 

 up the shaft ; for the leakage of air 

 downwards through the top whilst 

 the cage is in the act of opening or 

 closing the air-valve, and through 

 the small area that always remains 

 open, is found to be quite imma- 

 terial, and the surplus ventilating 

 power of the fan is amply sufficient 

 to provide against it. 



In the original construction a 

 more perfect air- valve was supposed 

 to be requisite, and was provided by the inclined flaps s s, which are fixed just above 

 the horizontal tunnel i. These are fitted closely together, leaving only a small opening 

 in the centre for the chain to pass through, and were intended to be opened by the 

 ascending cage coming in contact with them, closing again directly by means of 

 balance weights before the air-valve R at the top of the shaft was opened, so as to pre- 



