THE STEAM-ENGINE. 157 



be others which do not occur to me at the moment. Consider 

 ing the principles of the double-cylinder engine it is readily 

 seen that if the steam were simply used throughout the full 

 stroke in the small cylinder and were then turned into the 

 large one, there would not be any reduction of initial strain 

 whatever on the piston of the large cylinder nor on the 

 parts attached thereto, because if there were no loss of pres- 

 sure, although there would be a constant decrease as the 

 steam expanded, yet, at the moment of turning it on there 

 would be the same pressure on the low-pressure piston as if 

 you turned the steam in directly from the boiler. But now- 

 a-days we expand in the high-pressure cylinder to a greater or 

 less degree, and thus the piston of the low-pressure cylinder 

 does receive much less strain than if the boiler steam were turned 

 upon it. This arrangement tends to diminish the pressure on 

 the low-pressure cylinder, and it tends also to equalize the 

 strain, producing a more equal tangential force, but these ad- 

 vantages are obtained at a very considerable cost. I am sorry 

 I have not got a diagram here, which would have shown you 

 practically why it is when you have a compound cylinder en- 

 gine you obtain no more work out of the two cylinders than 

 you would obtain if you put the same quantity of steam into 

 the low-pressure cylinder alone, and gave it the same amount 

 of expansion. I do not say the high-pressure cylinder is 

 not doing work, because it is, but that which it does is 

 abstracted from the low pressure cylinder, and in that way, 

 therefore, one has all the complication and expense of the 

 two cylinders, and the final result got out is no more than 

 if one had a single cylinder of the same size as the low- 

 pressure cylinder not as the . aggregate size of the two 

 and expanded the steam in that cylinder to just the same 

 extent. We have thus a very large amount of compli- 

 cation introduced into an engine for apparently a very 

 small end. However, it is so introduced, and it is the or- 

 dinary construction now-a-days in marine steam-engines. It is 

 said there is a utility in it, because one not only diminishes 

 the strain on the reciprocating parts, but the tangential forces 

 are equalised to a great extent. That- is true, but I want 

 you to see what the value of that is. I have here Diagram 10, 

 which shows by Fig. 1 an engine working without any ex- 

 pansion, and by the figure below it the tangential effect in 

 producing the motion on a crank. 



