Transportation on Land 311 



on page 312 for the expansive capacity of steam below atmos- 

 pheric pressure. 



The compound engine. The compound engine is becoming 

 familiar in locomotives. The triple and quadruple expansion 

 types are necessarily restricted to marine and stationary use. 

 The compound engine makes use of the steam in a series of 

 two cylinders. The used steam from the first cylinder is con- 

 ducted to, and made to perform work by greater expansion in, 

 the second cylinder. This is generally made possible by having 

 the second cylinder adapted to a lower temperature and pressure 

 than the first. So perfectly is this adjusted that each cylinder 

 gets its fraction of steam's expansion and so of its heat energy. 

 Compound and multiple expansion engines are designed so as 

 to divide the total energy of steam at a given pressure and tem- 

 perature into two, three, four, or five parts. The steam is al- 

 lowed to perform just so much work in each cylinder. When 

 superheated, it has of course greater energy, which it gives up 

 in its course through successive cylinders, or sections of turbines. 



Table of steam pressures (for reference). The table on page 312 

 gives a few typical cases of the relation of absolute pressure to 

 (a) volume of one pound of steam in cubic feet ; (6) latent heat of 

 steam in B. T. U. ; (c) the heat of water in B. T. U. ; and (d) the 

 boiling temperature of water in Fahrenheit scale. The heat of 

 water is the amount of heat added from 32 F. to 212 F., and not the 

 total quantity of heat which water may have above absolute zero or 

 273 C. Gage pressures are given for cases below atmospheric 

 pressure. For all above subtract 14.7 pounds from absolute pressure 

 to find gage pressure. 



Study of the data presented in the table enables one to see 

 more clearly the enormous quantity of heat energy which a given 

 mass of steam at a given pressure contains. It emphasizes the 

 advantages in making use of steam as a medium for the transfer 

 of heat. It should make clear also why steam is utilized at 

 very high pressures in the modern engine. An absolute pressure 

 of 240 pounds per square inch is not unusual to-day. This 



