The lower cylinder of the spring-balance sometimes carries an index or pointer, _t_, fig. 74, which protrudes through a slot in the cylinder _g_, and indicates the amount of pressure of the spring on a scale marked along the slot on the outside of the cylinder. If it is desired that the safety-valve should open when the steam pressure reaches 100 or any other number of pounds per square inch, the spring-balance is subjected to a tension which will bring an amount of pressure on the top of the safety-valve equal in pounds per square inch of its surface to that of the steam pressure desired.[38] [38] In loading a safety-valve allowance must always be made for the weight of the lever and the valve. There should always be some provision made which will render it impossible to increase the steam pressure beyond that which it has been determined that the boiler will safely bear. This is usually done by arranging one of the safety-valves with a lever, as shown in fig. 74, and the other without, like that in fig. 76. The latter is often covered and sealed or locked up, so as to be beyond the control of the locomotive runner. The safety-valves are usually fitted into conical seats, _S S_, figs. 74 and 76, so as to be perfectly steam-tight, and are made with wings or guides, _t_, _t_, the form of which is shown in the sectional plan _A_, figs. 74 and 76, under the valve. These guides are intended to keep the valves in the proper positions in relation to their seats. [Illustration: Fig. 76. Scale ¹⁄₈.] As soon as the steam pressure under the valves becomes greater than the pressure of the springs on top of them, the valves will be lifted up and the steam will escape until the pressure in the boiler is relieved. It will be seen, however, that although the surface of the valve which is exposed to the pressure of the steam is equal to the area of the opening for the valve, after it is lifted from its seat and the steam escapes all around the edge, a larger surface will be exposed to the pressure of the escaping steam. For this reason, it will be found that after a valve is opened steam will escape, or “_blow off_,” as it is termed, until the pressure is several pounds lower than it was when the valve first opened. Advantage has been taken of this fact in the valve shown in fig. 76, which is called the “Richardson” valve, which is now much used. The top of this valve is made larger in diameter, so as to expose more area to the escaping steam. Grooves are also made around the edge of the valve and the seat. These, it is claimed, produce some sort of reflex action of the steam, which keeps the valve open longer than it otherwise would be. [Illustration: Fig. 78.] [Illustration: Fig. 77.] QUESTION 146. _How is the steam pressure in the boiler indicated?_