204 



OF THE PARTS OF 



[SECT. vn. 



Valves 



Reciprocating 



Rising 



Sliding 



On an axis 



Rotary 



Flat valves. 



Conical 



Spherical. 



Flat 

 Cylindrical. 



Throttle valves. 

 Bettancourt's. 



Common. 

 Hornblower's. 



Bramah's. 

 Murray's. 

 Murdoch's. 



{Common. 

 Four-passaged. 

 Bramah's. 



Plate or flat I Regulator. 



433. The office of a valve or cock being to open or to close a passage in the 

 most perfect manner, and either instantly or progressively, as may be proper for 

 the object in view, it is evident that those which offer the least obstruction to the 

 passage, and are opened with the least force, are the best. Hence the convenience 

 of considering them in succession in this place, and referring them to their 

 respective uses. 



434. All the species of valves become more difficult to manage in proportion 

 as the apertures become larger. The area of the passage when open should be 

 rather more than equivalent to that of the narrowest part of the pipe, and by 

 comparing these areas the proportions of the valve boxes or apertures will be 

 easily found for each species. 



435. When valves, cocks, or sliders are to be moved to admit steam to a 

 steam engine, the motion should be as quick as circumstances will permit, so that 

 the passages may be wholly opened or wholly closed at the proper time with the 

 least delay ; for it may be easily shown that a considerable loss of effect arises from 

 valves opening or shutting with a slow motion. 



RISING VALVES. 



436. The common CLACK VALVE is one of the most simple : in its common form 

 it is a plate of leather a little larger than the valve aperture with a part of it fixed 



