All About Engines

How a Modern Engine Works 3 perature the pressure of the steam is just equal to that of the atmosphere. If the operation took place at the bottom of a mine the temperature of boiling water would be higher, because there is a greater thickness of air overhead ; and if it were carried out on the top of a mountain the temperature would be lower because the thickness of the atmosphere overhead would be less. The temperature just given for the boiling point is true for the neighbourhood of sea level and with the barometer at 30 inches. On heating water in a closed vessel the tempera- ture and pressure both increase, and for every tem- perature there is a corresponding pressure of steam. Thus at a temperature of 2120 Fahr, the pressure is 147 lb. per square inch, or equal to that of the atmosphere ; at 2930 Fahr, the pressure is 60'4 lb. per square inch ; at 320° Fahr, it is 101-9 lb. per square inch; at 3740 Fahr. 182'4 lb. per square inch ; and so on. As boilers are not constructed STEAM Fig. 2.—Lever safety-valve to stand more than certain definite pressures, and as a man cannot always be watching the pressure gauge, some device is needed to enable steam to escape when the pressure exceeds that which the boiler was in- tended to bear safely. Such a de- vice is shown in Fig. 2.