Compound Locomotives 87 



each carrying a different pressure. The 

 system having the highest pressure, 1,400 

 i,8oolb. per square inch, the variation 

 in pressure depending on the rate of firing, 

 is in the form of a " closed circuit " con- 

 sisting of a number of pipes which form 

 the sides, roof and back end of the fire-box. 

 These pipes are connected at the bottom 

 to a foundation ring, and at the top to 

 equalizing drums being expanded thereinto. 

 From the equalizing drums, pipes are led 

 to evaporating elements which are situated 

 in the high-pressure drum. 



This closed circuit is initially filled to a 

 pre-determined level with pure water, and 

 this latter is the medium by means of which 

 heat is transmitted from the fire-box to the 

 evaporating elements in the high-pressure 

 drum, which furnishes steam at 900 Ib. per 

 square inch for the high-pressure cylinder 

 of the locomotive. This drum is of nickel 

 steel, but is not in contact in any way with 

 the fire. It is fed by water drawn as required 

 from the low-pressure boiler by means of 

 a pump. 



