CHAPTER IX POWER FROM HEAT THE TERM “water power” is a misnomer. There is no power inherent in water. It is gravity that makes water fall in cataracts or flow down a river bed, and hence it is the force of gravity that is responsible for the turning of our turbines and Pelton wheels. Water is merely the medium through which the force of gravity acts. In the steam engine, water, again, serves as a medium and it is the energy of heat that is actually the working force. A closed vessel is partly filled with water and heat is applied to it. The water grows hotter and hotter until at a temperature of 212 degrees it begins to vaporize. The temperature of the water remains stationary until the space above the water is completely filled with steam at a pressure equal to that of normal air pressure. The steam gauge then registers zero. Continued application of heat raises the pressure of the steam and also raises the temperature of the water. When the steam gauge shows a pressure of 25 pounds per square inch, the water temperature is nearly 267 degrees; at 100 pounds it is over 337 degrees; at 200 pounds it is 387 degrees; at 500 pounds it is about 467 degrees. As steam is drawn off from the boiler a proportionate amount of water turns immediately into steam to take its place. If the boiler should burst, the entire mass of water would instantly flash into steam because its temperature is far above the boiling point at normal air pressure. That is why the explosion of a steam boiler is so violent. If the boiler were entirely filled with steam the effect of an explosion would not begin to be so destructive as if the boiler were half full of water. It is really the explosion of water that does such serious damage. When the steam pressure in a boiler corresponds to the temperature of the water it is said to be _saturated_. The water in a boiler boils violently when the steam is drawn off rapidly and tiny droplets of water are carried off in the steam, producing what is known as _wet steam_. Steam that carries no water particles in suspension is called _dry steam_. When the steam is heated above the temperature of the water by means of an auxiliary heating device or by some peculiar construction of the boiler, it is called _superheated steam_. In order to economize fuel it is highly important that as much water surface be exposed to the heat as possible. This may be done either by passing the heat in tubes through the water or by passing the water in tubes through the fire. Locomotive boilers are of the fire-tube type. The flaming gases of the furnace pass through the boiler through a series of tubes that lead to the stack. In the water-tube boilers it is highly important that a good and rapid circulation of water be maintained, otherwise there might be local generation of steam with serious consequences. THE GIFFARD INJECTOR [Illustration: FIG. 45.--INJECTOR FOR INTRODUCING WATER INTO A STEAM BOILER]