138 THE PRODUCTION AND USE OF HEAT 



The principle of the pop valve is shown in Fig. 101. The 

 valve in this case is held down by a spiral spring. The top 

 of the spring rests against a metal plate, P. This plate may 

 be forced farther down by turning down the screw, S. The 

 spring then holds the valve down against a greater pressure. 

 In each type, the valve opens and allows some steam to escape 

 as soon as the pressure within the boiler exceeds the amount 

 for which the valve is set. 



159. The Water Gauge. The purpose of the water gauge is 

 to enable the operator or engineer to see exactly how high the 

 water stands in the boiler. The gauge is merely a strong glass 

 tube mounted on the side of the boiler at the height at which 

 the water should stand. This glass tube is so connected at 

 both its top and its bottom that the water stands within it at 

 the same height as it stands within the boiler. This gauge is of 

 the greatest importance, for the person in charge must never 

 permit the water within the boiler to get lower than the 

 bottom of the gauge. The danger from an explosion is very 

 great if the water is permitted to get too low in the boiler. 

 Point out the water gauges on the boilers shown in the illustra- 

 tions (Figs. 96, 358, 359 and 360). 



160. The Temperature-pressure Curve. Many of the 

 facts of science, especially those of physics, chemistry, and 

 engineering are shown in their true relations much more 

 readily and more clearly by using DIAGRAMS and CURVES 

 than by merely giving the figures. Figure 102 is a curve show- 

 ing the relation of temperature to pressure in the case of boiling 

 water. The various temperatures of the steam are shown 

 along the left-hand margin of the figure while the corresponding 

 pressures are indicated along the bottom of the figure. The 

 curve is a line connecting all the points of temperature and the 

 corresponding pressures. For example, we have seen that 

 water boils at 100C., or 212F., when the pressure is 14.7 

 Ib. per sq. in. This fact is indicated by the point, A, on the 

 curve. Likewise the boiling point of water is 50C., or 122F., 

 when the pressure is 1.78 Ib. per sq. in. This fact is shown 



