Some steam-heating systems have a check valve in the pipe which returns water to the boiler. This valve permits water to flow thru it in but one direction; that is, toward the boiler. This prevents a rush of water from the boiler to the radiators. Steam furnaces, also, are often equipped with another safety-valve device, which is a plug of metal which melts at a rather low temperature and is placed in the boiler directly over the fire. If the water line in the boiler falls low, this plug melts and steam from the boiler puts out the fire, thus saving the furnace from damage. However, melting out the plug makes much work both in replacing the plug and in cleaning the fire box to rebuild the fire, so that it should not be depended upon to regulate the heat in the boiler. Knocking in steam radiators occurs most often in those systems using the inlet steam pipe for the return of the water which has formed as a result of condensation. It is caused by water accumulating at some point and the steam coming up the pipe, violently forcing it back into the radiator. This only reaches a danger point in systems which do not have pipes of the proper size, or when the pipes do not slope gradually downward, so that all the water may flow back to the furnace. On cold days, there will be some knocking in a steam radiator when it is being heated in the morning. A two-pipe system, while it is somewhat more expensive, is less subject to this trouble. =71. Steam Gages.= Steam gages (_B_, Fig. 35) are devices for indicating the pressure of steam within an inclosure. They are a kind of spring balance. When the pressure of the steam increases, it pushes up on the spring, and this turns the hand of the indicator, which shows the number of pounds of pressure that the steam is exerting on the inside of the boiler or container. =72. Safety Valve.= A safety valve (Fig. 36 and _A_, Fig. 38) consists of a small opening to a boiler over which is a weight. When steam is developed until it makes enough pressure on the inside of the valve to raise this weight, some of the steam escapes, thus lowering the pressure on the inside until the weight falls back into place. Never let anything interfere with the action of safety valves. [Illustration: FIG. 38. Heating plant showing safety valve.] Most safety valves have the weight attached to a lever which has a movable weight on it so that the position of the weight on the lever makes a difference in the number of pounds of pressure required to open the valve. By means of this device, the temperature of the inside of the boiler can be kept at one heat or another as desired, since this temperature increases or decreases with the pressure under which the steam is held.