THE STEAM-ENGINE. 453 



THE STEAM-ENGINE. 



(THIRD LECTURE.) 



IN the single-acting engine, the force of the piston acted on the beam only 

 during its descent ; and this force was transmitted from the piston to the beam, 

 as we have seen, by a flexible chain, extended from the end of the piston-rod, 

 and playing upon the arch head of the beam. In the double-acting engine, 

 however, the force of the steam pressing the piston upward must likewise be 

 transmitted to the beam, so as to drive the latter upward while the piston as- 

 cends. This action could not be accomplished by a chain connecting the 

 piston with the arch head of the beam. 



Where the mechanical action to be transmitted is a pull, and not a push, a 

 flexible chain, cord, or strap, is sufficient ; but if a push or thrust is required 

 to be transmitted, then the flexibility of the medium of mechanical communica- 

 tion afforded by a chain renders it inapplicable. In the double-acting engine, 

 during the descent, the piston-rod still pulls the beam down ; and so far a chain 

 connecting the piston-rod with the beam would be sufficient to transmit the 

 action of the one to the other ; but in the ascent the beam no longer pulls up 

 the piston-rod, but is pushed up by it. A chain from the piston-rod to the 

 arch head, as described in the single-acting engine, would fail to transmit this 

 force. If such a chain were used with the double engine, where there is no 

 counter-weight on the opposite end of the beam, the consequence would be, 

 that in the ascent of the piston the chain would slacken, and the beam would 

 still remain depressed. It is therefore necessary that some other mechanical 

 connexion be contrived between the piston-rod and the beam, of such a nature 

 that in the descent the piston-rod may pull the beam down, and may push it up 

 in the ascent. 



Watt first proposed to effect ttis by attaching to the end of the piston-rod a 

 straight rack, faced with teeth, which should work in corresponding teeth 

 raised on the arch head of the beam, as represented in fig. 16. If his im- 

 proved steam-engines required no further precision of operation and construe- 



