CHAP, vii.1 VARIOUS TYPES OF STEAM-ENGINES. 425 



CHAPTEE VII. 



VABIOUS TYPES OF STEAM-ENGINES. 



I. WATT'S BEAM-ENGINE. 



WE come now to the machinery for transmission. We have to 

 examine how the motion either of the beam or the shaft 

 is utilised by the working of the slide-valve and of the feed and 

 exhausting pump. 



To the shaft of the engine is fixed an excentric seen at dd in 

 Fig. 299, the function of which is to produce the alternate motion of 

 the slide-valve. It is very easy to explain how this result is obtained. 

 The excentric is formed of a circular metallic disc, which is pierced 

 by the shaft at a point which is not its centre. Its motion of rota- 

 tion involves that of a collar or band in connection with a long 

 metallic triangle. Now the extremity of the latter is attached to 



one of the arms of a bent lever, the other arm of which carries the 







rod of the slide-valve. The oscillating motion of the lever produced 

 by the rotation of the excentric gives rise to an alternating vertical 

 motion of the rod, and the slide-valve works as we have seen above. 



Figure 299 represents the beam-engine, as it came from the hands 

 of Watt, with all the improvements that that illustrious mechanician 

 successively made in it ; it gives the reader a general view of the 

 various mechanical arrangements with respect to the distribution and 

 transmission that we have had to describe separately and in detail. 

 It remains for us now to show how the various pumps, which we 

 have mentioned in our description of the engine, work. H is the 

 condenser which is bathed ill a cistern of cold water RR, and which 

 receives water from that cistern by the pipe t. Since the conden- 

 sation of the steam cannot take place without its giving up to the 



