realizing the Advantages of the Air-Engine. 25 



plunger and the receiver, are a number of parallel vertical 

 plates of metal or glass, with narrow interstices between them, 

 through which the air must pass on its way up or down. This 

 system of plates is called the Economizer or Regenerator ; 

 its object being one which has already been explained in ar- 

 ticle 8, namely to store up the heat given out by the air dur- 

 ing the process C, when its temperature is being lowered, and 

 to give back the same heat to the air during the process A, so 

 as to raise its temperature. Lower still, the receiver has an 

 internal false bottom, pierced with many small holes, through 

 which also the air must pass, and whose effect is to bring 

 every part of the air into close contact with the heated iron 

 bottom of the receiver. Suppose, further, that this receiver 

 communicates at its top, through a sufficiently wide passage or 

 nozzle, with the lower end of a working cylinder containing 

 the piston ; the receiver and cylinder are, in the first place, 

 filled with compressed air, of any required density, by means 

 of a small forcing-pump. As the same mass of air is used 

 over and over again, this pump has to be subsequently worked 

 to no further extent than is necessary to supply the loss of air 

 by leakage, which has always been found to be extremely 

 small. A pump is also required to keep up a stream of cold 

 water through the coil of tubes before mentioned. 



19. Mode of operation of Stirling' s Air-Engine. — Suppose 

 the piston to be at the bottom of the cylinder, and the plunger 

 at the bottom of the receiver, the mass of air in the receiver is 

 now at the top amongst and near the cold-water tubes, and its 

 temperature is low. Let the plunger now be partially raised, 

 part of the air is forced down through the economizer into the 

 space between the outer and inner bottoms of the receiver, and 

 through the holes of the inner bottom, into the space below 

 the plunger. In passing over the heated bottom of the receiver, 

 it has, in the first place, its temperature raised by the reception 

 of heat from the furnace. At this point the cycle of processes 

 formerly described may be held to begin. 



Process B. — The air below the plunger receives an addi- 

 tional supply of heat from the furnace, which disappears in 

 expanding it. The air below the plunger, in the act of ex- 

 panding, lifts up the plunger and the mass of air above it, 



