164 LECTURES TO SCIENCE TEACHERS. 



becomes lower in temperature, the heat which has been 

 absorbed by the walls of the cylinder is given out, and the 

 dew which has been deposited upon them evaporates, and we 

 thus get a final pressure when there is no jacket, very much 

 in excess of that which ought to arise from the quantity of 

 steam that appears to have been put into the cylinder. You 

 will always find in a high expansive non-jacketed engine that 

 the final pressure is very much in excess of that which 

 apparently ought to be there, but if we take into account the 

 steam which has been condensed, you will readily see where 

 this excess of final pressure comes from, Now, had not this 

 portion been condensed, but remained as steam, instead of 

 obtaining only the useful effect represented by the full white 

 line on that diagram, we should have the useful effect repre- 

 sented by the middle dotted white line, which at its termi- 

 nation would coincide with the full white line, but in any 

 other position is outside of it. This difference shows the 

 excess of useful effect obtained by the aid of the steam 

 jacket. You also get the steam superheated by the steam in 

 the jacket during the progress of the piston. 



I have spoken of the variations of tangential force, but 

 those are variations which occur while the resistance to the 

 engine is constant; we know that in factory engines, however, 

 the resistance varies very considerably, and therefore one is 

 compelled to have some kind of governor. I wish time 

 admitted of my going into the question, because we have 

 here an interesting series of governors. In the Watt governor 

 the number of revolutions at which the balls will fly out 

 depends upon the vertical height between the point where 

 the arms cross each other in the centre of the spindle and 

 the plane where the revolutions of the balls may be ; when 

 the load upon the engine is light, in order that the throttle 

 valve may be sufficiently nearly closed, the balls require to 

 be right up, and to maintain them there, the engine must 

 run at a pace in excess of that at which it runs when it is 

 heavily loaded. 



From this cause the Watt governor, where the arms 

 vibrate on a pin in the axis of the vertical spindle, is not 

 such a true controller of the engine as to keep it at 

 regular speeds. The model by Mr. Jeremiah Head I have 

 here illustrates this very welL Here is another very common 

 form of governor which is extremely bad, where the pivot of 





