THE STEAM-ENGINE. 167 



above atmosphere, without any expansion or condensation. 

 The quantity of steam employed, if there were no wire- 

 drawing, would be expressed by the area of Fig. I ; but 

 suppose now we wire-draw, and put a pressure of 37 Ibs. on 

 the piston at the beginning, then the line of pressure may 

 be represented by Fig. 2, and we have a reduced consump- 

 tion of steam as there shown. Supposing now we wire-draw 

 still further, and put a pressure of 57 Ibs. on the piston at the 

 beginning, we then reduce the consumption of steam to the 

 point shown on the third figure, and it really, therefore, 

 is a fact that where you have a bad engine you will do good 

 by wire-drawing. Do not understand me that it is a thing to 

 be used in lieu of proper expansion, but merely as saying 

 that so far from being, as is popularly supposed, a detriment, 

 in every instance, it is, in the case of a bad steam-engine, a 

 positive advantage. Again, take an ordinary engine without 

 expansion, and look at the enormous advantage which will 

 result from putting even a slight lap on the valve. Where 

 the slide travels about four times the amount of the steam 

 opening, you can. with a single slide, cut off at three-fourths of 

 the stroke, and by that arrangement you clearly introduce 

 into the cylinder only three-fourths of the steam, while you 

 have not diminished the work by even ^. Therefore, 

 in every bad engine, it is very easy for the first mechanical 

 engineer in the neighbourhood to do his part towards effecting 

 some saving in our store of coal. 



Engines should be constantly examined by the aid of the 

 steam-engine indicator. The original indicator as devised by 

 "Watt is shown by the diagram on the wall, with the re- 

 ciprocating card travelling at right angles to the path of the 

 pencil caused to move by the pressure of the steam. This 

 indicator has been largely improved by Richards, as shown 

 in the further diagram. The improvement has consisted 

 in diminishing the piston stroke, and multiplying it by 

 means of levers, so that the momentum of the indicator 

 is greatly reduced. In that way, and without any alteration 

 of the indicator as ordinarily sold, it is possible to take 

 thoroughly good diagrams of engines running up to 300 revo- 

 lutions a minute. It fell to my lot, however, to have to take 

 a diagram from one of Thorneycroft's quick steam launches 

 running at 600 revolutions a minute, and I was very desirous, 

 for the purpose of a paper I was writing, of obtaining the vary- 



