VALVES. 



tendency to close it, the pressure on the other side Jias a tendency 

 to open it. 



As in all forms of pump the water is required to be moved upwards, 

 all the valves necessarily open upwards and close downwards. 



Fig. 7. 



There are several varieties of form. 



10. The clack valve is like the lid of a box (fig. 8). 

 upwards, playing upon a hinge, and when 

 the water presses it downwards it is closed. 



The single clack valve is the most simple 

 example of the class. It is usually constructed 

 by attaching to a plate of metal larger 

 than the aperture which the valve is intended to stop, a piece 

 of leather, and to the under side of this leather another piece of 

 metal smaller than the aperture. The leather extending on one 

 side beyond the larger metallic plate, and being flexible, forms 

 the hinge on which the valve plays. Such a valve is usually 

 closed by its own weight, and opened by the pressure of the fluid 

 which passes through it. It is also held closed more firmly by 

 the pressure of the fluid whose return it is intended to obstruct. 



The extent to which such a valve should be capable of -pig. 9. 

 opening, ought to be such that the aperture produced by 

 it shall be equal to the aperture which it stops. This will 

 be effected if the angle through which it rises be about 30. 

 A double clack consists of two semicircular plates, 

 having the hinges on the diameters of the semi- 

 oircles, as represented in fig. 9. 



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