1880.] Sequel to the ' Thunderer ' Gun Explosion. 317 



experimenting with it in order to lay the results of the experiments 

 before the Institution of Mechanical Engineers. In making these 

 experiments, 1 found to my astonishment that the pressure of steam in 

 the cylinder was greater tlian that in the boiler from which the steam 

 bad set out. Had tlie difference] been only two or three pounds I 

 should have attributed it to the excess needed to force the steam back 

 into the boiler, but the observed difference was as much as 30 lbs., and 

 moreover it was obvious that the true limit of pressure had not been 

 reached, but that tlie further rise of the indicator had been stopped, 

 the spring then in the indicator having been compressed to the full 

 extent of its range. My first impression was that the indicator was 

 out of order, although in its immediately previous use, on the engine 

 when running in forward gear in the ordinary manner, it had accorded 

 so completely with the pressure of steam in the boiler as to render 

 such an assumption very improbable ; but on testing the indicator it 

 was found to be quite accurate. I then had to cast about for the 

 cause of the phenomenon of the excess, which was revealed by the indi- 

 cator diagrams, enlarged coj^ies of which diagram (6) are on the wall. 

 To explain this cause I must refer you to the skeleton diagram (7) 

 of the locomotive. From this it will be seen that the steam was taken 

 from one end of the boiler, and was then conducted by a pipe the 

 whole length of the boiler to the cylinders. When working in reverse 

 gear the steam is suddenly admitted from the boilers into the cylinder 

 when the piston is about half-way along the cylinder ; as a result 

 the steam is set in very rapid motion in the long pipe, and then upon 

 the cylinder being filled, its motion is resisted, and the stored-up work 

 in the weight of steam travelling at the high velocity along this pipe 

 is sufiicient to cause the pressure to rise in the cylinder to such a 

 point above the pressure in the boiler as will absorb the " work " in 

 the steam in the pipe. 



I think I shall be able to illustrate that which I mean by the 

 little apparatus I have here. This is a gas-holder, now, however, 

 filled with air, giving, as you will see by the gauge, a statical pressure 

 of nine inches of water, which I must ask you to accept as the 

 equivalent of the pressure of the steam in the boiler. From the 

 gas-holder a horizontal pipe (the equivalent of the long pipe in 

 the boiler) proceeds. This pipe terminates in a vessel which is the 

 equivalent of the cylinder in the locomotive. The pipe is shut off 

 from the gas-holder by a stop-cock, and is shut oflf from the pressure 

 gauge, placed at its end, by a little valve opening outwards towards 

 that gauge, the gauge itself deriving its pressure from another pipe 

 having a small hole of connection. On the sudden opening of the 

 stop-cock, I think you will find that the air on rushing along the 

 pipe and filling the vessel at the end of it, will not be content with 

 producing a pressure in the vessel equal to that in the holder, but will 

 by virtue of the stored-up work in the air in motion produce a pressure 

 in the vessel sufficiently higher than that in the holder to open the 

 valve against the gas-holder pressure, and to raise the water in the 



