=The Safety Valve.=--All steam boilers should be provided with safety valves as a safeguard against excessive steam pressures. Of the various types of safety valves, that known as the _pop-valve_ is most commonly used on house-heating boilers. It is indicated at _W_ in Fig. 18 and is shown in section in Fig. 21. The part _A_ is screwed into the top of the boiler at any convenient place. The pressure of the spring _C_ holds the valve _B_ on its seat until the internal pressure reaches a certain intensity at which the valve is set, when it opens and allows the excess steam to escape. When the pressure is reduced, the spring forces the valve back on its seat. The handle _D_ permits the valve to be lifted at any time as an assurance that it is in working order. This should be done occasionally, as the valve may stick to the seat after long standing and allow the pressure to rise above the point at which it should “pop.” The valve may be set to “blow off” at any desired pressure by the adjusting piece _E_. House-heating boilers generally have their safety valves set to blow off at 8 or 10 pounds. =The Draft Regulator.=--As a means of automatic control of the steam pressure, the draft regulator is frequently used to so govern the fire that when a certain steam pressure is reached, the direct draft will be automatically closed and the check-draft damper opened. The draft regulator is shown in place at _D_ in Fig. 18, and will also be found in Fig. 16. A detailed description of the regulator will be found on pages 60 and 61. RULE FOR PROPORTIONING RADIATORS Rules for determining the amount of radiating surface that will be required to satisfactorily heat a building to 70°F. regardless of weather conditions are entirely empirical, that is, they are derived from experience. It is evident that no definite rule can be established that will take into account the method of building construction, the kind and amount of materials that make up the walls and the quality of workmanship employed. These variable quantities coupled with the changing climatic conditions of temperature and wind velocity produce a complication that cannot be overcome in a formula that will give exact results. Many rules are in use for this purpose, no two of which give exactly the same results when applied to a problem. A common practice is to apply one of the rules in use and then under conditions of exceptional exposure, to add to the amount thus calculated as experience may dictate. The following rule by Professor R. G. Carpenter of Cornell University was taken from a handbook published by the J. L. Mott Iron Works of New York. This company manufactures and deals in all kinds of apparatus entering into steam and hot-water heating and the rule is given as one that has produced satisfactory results.