When water turns into steam a certain amount of heat is absorbed, which does not show in the thermometer. This is known as latent heat. If cold water is placed in a kettle and is then heated the thermometer will gradually rise until the water reaches the boiling point. Then there will be no further rise of temperature, although heat is still applied to the kettle, until all the water is turned into steam, after which the thermometer will begin to show a rise of temperature, showing that the steam is beginning to grow sensibly hotter. In the engine, shown in Figure 54, a boiler pressure of 120 pounds gauge pressure is maintained, and therefore water will not boil until it reaches 350 degrees Fahrenheit. The water in the water jacket registers 350 degrees and is therefore far hotter than it could possibly be in the open atmosphere. However, the heat from the combustion does not raise the temperature of the water, but expends its energy in converting the water into steam. The cylinder wall is kept at a temperature of at least 350 degrees all the time, so that the air that enters the cylinder gathers heat from the cylinder as well as from its own compression, and its temperature at the end of the compression stroke is higher than it would be in a cold cylinder, thereby insuring the ignition of the fuel when it is sprayed in. The Still engine can also be used as a common gasoline or gas engine of either two or four cycle and the efficiency in such types is from 31 to 33 per cent. This is much better than the best airplane engines, which show an efficiency under 27 per cent. Although 41 per cent for the best Still heavy-oil engine is a remarkable accomplishment, yet it does not begin to compare with the efficiency of the best water turbines and Pelton wheels, which turn into useful power from 75 to 87 per cent of the kinetic energy in the water that drives them. CHAPTER XI WHEN COAL AND OIL ARE EXHAUSTED IN PREVIOUS chapters we have indulged in a great deal of historic retrospect. It may be well at this point, while we are dealing with the subject of power, to look into the future and the prospects that it holds out to us.