CHAP. VI.] 



THE STEAM-ENGINE. 



417 



adopted. But before insisting on the advantages it presents, or indi- 

 cating the economy of steam or of fuel which expansion secures, we 

 must show by what modification of the distributing machinery it 

 may be accomplished. 



Here again, if we were intending to write a complete treatise on 

 the steam-engine, we should have to describe the various systems of 

 expansion. It will suffice however for the end in view to give an 

 idea of one or two of the most important. 



We will commence with the system of expansion called Clapey- 

 ron's, because its arrangement is due to that eminent engineer. 



It consists in a simple modification of the slide-valve, or rather of 

 the breadth of the bands which cover the ports. Instead of giving 

 them the exact breadth of each port, they are made larger. The 

 ledges ab, a'b' , cd, c'd', inside and outside form what is called the laps 

 of the slide-valve, because it is the object of these overlaps to shorten 

 the time of admission of the steam into the cylinder through each 



FIG. 293. Clapeyron's expansion system : slide-valve with laps. 



of the two ports. It would be necessary to enter into too long and 

 technical details to follow the motion of the expansion-slide valve 

 through all its phases, and to show clearly what is the action of the 

 steam in each of these phases. But we can sum up the whole action 

 by saying that each introduction of the steam into the cylinder gives 

 rise to four successive periods, which we will characterize. 



In the first period there is the admission of the steam, which 

 works during that time at its full pressure, that is, with the pressure 

 of the steam in the boiler, after which a steam-entrance port is closed. 



In the second period there is the expansion of the steam admitted, 

 which then works with a decreasing force until the moment when the 

 steam- exhaust port opens. 



The escape of the steam occupies the third period, but since from 

 the existence of the laps the escape ceases before the piston has 

 reached the bottom of the cylinder, there remains a certain quantity 



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