In order to measure positively all pressures above a vacuum, the normal atmosphere is 14.7 pounds; all pressures above that point are continued on the same scale, thus: Gage pressure 0 = 14.7 absolute Gage pressure 10 = 10 + 14.7 = 24.7 absolute Gage pressure 20 = 20 + 14.7 = 34.7 absolute Absolute pressures are, therefore, those of the gage plus the additional amount due to the atmosphere. All references to pressure in this work are intended to indicate gage pressure unless specifically mentioned as absolute pressure. Steam heating as applied to buildings may be considered under two general methods: the pressure system in which steam under pressure above the atmosphere is utilized to procure circulation; and the vacuum system in which the steam is used at a pressure below that of the atmosphere. Each of these systems is used under a great variety of conditions, and to some is applied specific names but the principle of operation is very much the same in all of a single class. Steam heating plants are now seldom installed in the average home but they are very much employed in apartment houses and the larger residences. In large buildings and in groups of buildings heated from a central point, steam is used for heating almost exclusively. The type of plant employed for any given condition will depend on the architecture of the buildings and their surroundings. In very large buildings and in groups of buildings, the vacuum system is very generally employed. This system has, as a special field of heating, the elaborate plants required in large units. The low-pressure gravity system of heating is used in buildings of moderate size, large residences, schools, churches, apartment houses, and the like. Under this form of steam heating is to be included vapor heating systems. This is the same as the low-pressure plant except that it operates under pressure only slightly above the atmosphere and possesses features that frequently recommend its use over any other form of steam heating. The term vapor heating is used to distinguish it from the low-pressure system. =The low-pressure gravity system=, with which we are most concerned, takes its name from the conditions under which it works. The low pressure refers to the pressure of the steam in the boiler, which is generally 3 or 4 pounds; and since the water of condensation flows back to the boiler by reason of gravity, it is a gravity system. The placing of the pipes which are to carry the steam to the radiators and return the water of condensation to the boiler may consist of one or both of two standard arrangements. They are known as the _single-pipe system_ and the _two-pipe system_. [Illustration: FIG. 1.--Diagram of a gravity system steam heating plant.]