58 



VAPORIZATION. 



piston is elevated four feet, or reaches the top of the cylinder, 

 the tension of the steam below it will still be or one atmos- 

 phere. The piston has, therefore, been raised to a height of 

 three feet, with a force progressively diminishing from four 

 atmospheres to one, or with an average force of two atmos- 

 pheres, by means of a power acquired without any consumption 

 of steam, but by the expansion merely of steam that had 

 already produced its usual effect. 



The boiler used to produce the steam is constructed of diffe- 

 rent forms. The common wagon boiler is represented in 

 Figure 1. The heated air from the fire below the boiler, after 

 passing under its whole length, is brought back, before pass- 

 ing to the chimney, by flues, 

 o, 0, in order that what heat 

 the air still contains, may 

 be imparted to the sides of 

 the boiler. The water is sup- 

 plied in proper quantity to 

 the boiler, and kept at a con- 

 stant level from a fountain- 

 head g, by a tube descend- 

 ing into the boiler from a 

 box above it c v. The mouth 

 of this tube is closed by a 

 valve, which is kept shut by 

 pressure from the lever a b, 

 loaded at a. But to the other limb b e of the lever an iron-rod 

 is attached, which descends into the boiler, and is fixed to a 

 piece of wood/", which floats upon the surface of the water. 

 When the level of the water is lowered, / falls with it, and 

 occasions the valve above to be opened and water to^ flow into 

 the boiler. 



The cylinder boiler, of which 

 a section is given in Figure 2, 

 is preferred as the most eco- 

 nomical, for the great steam- 

 engines at the Cornish mines. 

 It consists of two cylinders, 

 one within the other, the 

 smaller cylinder containing 

 the fire, and the space be- 

 tween the two cylinders being 

 occupied by the water. The 



FIG. 2. 



