upwards in id tht* valve down wank. By the time the piston 

 hun reached the top ol iU stroke Ui* ill have moved 



so far down as to partly uncover the team port 8,, and admit 

 steam from the valve easing C through 8, and the passage P, 

 iie piston. The presmre of this steam o 



I..* n. \\hiletheabove 



has been going on, the port 8 t will have become on- 



covered, and the hollow |*rt of the valve will be opposite both 



the steam port S u><l the exhaust port E, so that the steam 



'.' under side of the piston, and hi, h i,, r , ,,| th.- piston 



up, can now escape by the passage P Sl the steam port S,, ami 



< exhaust outlet O, and thence into 



the atmo*) i non-condensing engine. ., r into the 



-using engine, or into another cy I inder 

 and engine. he piston has performed 



a cert ml stroke, the valve, which has 



been i .wards, will commence to move upwards, 



and v ! . > reached a o will cover the port 



i h-.n ..tr the supply of steam to the top of the piston. 

 It is generally arranged that the steam shall be cut off before 

 the piston reaches the end of the stroke. When the piston 

 reaches the bottom of its lias moved far 



enough up to uncover the port 8, and admit steam t 

 bottom of the jist. uncover the port 8, and allow 



the steam to escape in-m th.- t<>|. ..i the piston through the 

 passage P,, the port 8,, the port ..utl.-t <>. In this 



\\;.v ih.- piston si nsoved up ind doi - in th.- . \ ttmte 



ve is attached to a \ ts as 



h<!r in the valve through whit h th*- s|>indle passes 

 being oval-shaped to {N>nnit of the valve adjusting itself so as 

 to always press on its seat 



When the valve is in it> ini<Klle |- .nerally more 



than covers the steam ports. The amount win. h th.- 



lie steam port on the outside, the valve being 

 in its middle position, is called the outMt lap of the N 

 .n,l the amount whu-h it projects on the inside is called the 

 iMMlr .rn the term lap b used without any qualific* 



iu lap is to be understood. In fig. 69 it will !- 



