136 THE PRODUCTION AND USE OF HEAT 



In the second column is given the ABSOLUTE PRESSURE, 

 the actual pressure on the inside surface of the boiler, in pounds 

 per square inch. 



In the third column is given the absolute pressure in at- 

 mospheres. It will be noted that in the first half of the table 

 the pressures are less than that of the atmosphere and in 

 the last half they are greater than that of the atmosphere. 



In the fourth column the gauge pressure is given. The gauge 

 pressure shows the difference in pressure on the inside and the 

 outside surfaces of the boiler. It will be noted that the 

 VACUUM GAUGE reading is given for all pressures less than 1 

 atmosphere and that the PRESSURE GAUGE reading is given 

 for all pressures greater than 1 atmosphere. It will also be 

 noted that when the boiling point is 100C., or 212F., both 

 gauges read zero. The vacuum gauge records the amount 

 of the vacuum, or the decrease in pressure below 1 atmos- 

 phere; the pressure gauge records the amount of pressure 

 above 1 atmosphere. If we had determined the pressure 

 in Ex. 40, we should have used the vacuum gauge because 

 the pressure was less than 1 atmosphere. 



With the exception of steam boilers used in connection with 

 VACUUM SYSTEMS of heating (Art. 163), practically all steam 

 boilers carry pressures greater than 1 atmosphere. All 

 steam boilers are therefore equipped with three safety de- 

 vices, a pressure gauge (Art. 157), a safety valve (Art. 158), 

 and a water gauge (Art. 159), for it is necessary, first, that the 

 operator shall be able to see at once what pressure the boiler 

 is carrying, second, that under no condition shall the steam 

 pressure become greater than that which the boiler is intended 

 to carry, and third, that the water in the boiler never gets 

 below a certain level. 



157. The Pressure Gauge. The purpose of the pressure 

 gauge is to indicate the steam pressure on each square inch of 

 the boiler. Its essential parts are: (1) A curved, somewhat 

 flexible, metallic tube, A, Fig. 99; this tube is connected by 

 a small pipe to the boiler; (2) hinged to the free end of 



