THE STEAM-ENGINE. 163 



we cannot work with the expansive engines to any good 

 purpose without a steam jacket, and I will endeavour to show 

 you how that is. I have here (Diagram 12) a section of a 

 cylinder with a steam jacket round it, and this diagram will, 

 I trust, explain why the steam jacket is beneficial. Assume 

 the cylinder to be without a steam jacket, and that then you 



DIAGRAM 12. 



have high pressure and proportionately hot steam let into that 

 cylinder, the immediate effect is to condense a portion of the 

 steam and to make it into water. It disappears as steam, 

 and the indicator diagram will only inform us, we had received 

 the steam up to a certain point, whereas in truth another 

 portion has been received but, having been condensed, does 

 not appear on the diagram. As the steam expands and 



M 2 



