416 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK iv. 



cylinder by its two plane ends, where the steam-entrance ports open. 

 The steam which comes from the boiler by the opening v can always 

 circulate round the slide-valve without obtaining access to either of its 

 extremities ; these, on the contrary, are always in free communication 

 with the condenser. The two plane ends of the slide-valve in their 

 motion to and fro allow each of the steam-entrance ports in turn to 

 receive the steam from the boiler, while the steam that has done its 

 work upon the piston passes out by the other port, and is condensed 

 in the condenser or the open air. 



In each of these methods of distribution it is easy to understand 

 the corresponding motions of the piston, slide-valves, and clacks in 

 their different phases. 



III. EXPANSION OF THE STEAM. 



In giving an account of the piston and the arrangements for the 

 distribution of the steam, it will be seen that the ports are sometimes 

 entirely uncovered, and sometimes entirely free. From which it 

 follows that the steam of the boiler pours with its full force upon 

 each face of the piston during the whole time of its motion ; this is 

 expressed by saying that the steam works at full pressure. 



At first no other way of letting the steam act was known ; but 

 Watt, whose name is found associated with all the principal dis- 

 coveries which have transformed the primitive steam-engine, found 

 that there was a double advantage in giving access to the steam to 

 the piston during a portion only of the course of the piston. The 

 result was first a much greater regularity in the motion itself, and 

 secondly for the same amount of work a notable economy of steam, 

 and consequently of fuel. 



If the steam, for example, is introduced during the first third only 

 of the course of the piston, it continues still to act upon it ; . but 

 since the space it occupies continues to enlarge until the end, it acts 

 by expanding, like a spring in opening, so that its force diminishes up 

 to the end of the stroke of the piston. The steam is then said to 

 work with expansion. 



This 'mode of action of the steam is now almost universally 



