Fig. 1 shows a diagram of a single-pipe system in its simplest form. In the figure the pipe marked _supply and return_, connects the boiler with the radiators. From the vertical pipe called a _riser_, the steam is taken to the radiators through branch pipes that all slope toward the riser, so that the water of condensation may readily flow back into the boiler. The water of condensation, returning to the boiler, must under this condition, flow in a direction contrary to the course of the steam supplying the radiators. In Fig. 2 is given a simple application of this system. A single pipe from the top of the boiler, in the basement, marked _supply and return pipe_, connects with one radiator on the floor above. The radiator and all of the connecting pipes are set to drain the water of condensation into the boiler. [Illustration: FIG. 2.--A simple form of steam heating plant. The furnace fire is controlled by a thermostat and a damper regulator.] When the valve is opened to admit steam to the radiator, the air vent must also be opened to allow the escape of the contained air. The steam will not diffuse with the air in the radiator and unless the air is allowed to escape, the steam will not enter. As the steam enters the cold radiator, it is rapidly condensed, and collects on the walls in the form of dew, at the same time giving up its latent heat. The heat is liberated as condensation takes place, and as the dew forms on the radiator walls the heat is conducted directly to the iron. The water runs to the bottom of the radiator and then through the pipes; back to the boiler. The water occupies but relatively a little space and may return through the same pipe, while more steam is entering the radiator. As the steam condenses in the radiator, its reduction in volume tends to reduce the pressure and thus aids additional steam from the boiler to enter. In this manner a constant supply of heat enters the radiator in the form of steam which when condensed goes back to the boiler at a temperature very near the boiling point to be revaporized. It should be kept in mind that it is the heat of vaporization, not the temperature of the steam that is utilized in the radiator, and that the heat of vaporization is the vehicle of transfer. The water returning to the boiler may be at the boiling point and the steam supplying the heat to the radiators may be at the same temperature. [Illustration: FIG. 3.--A gravity system steam heating plant of two radiators. The furnace is governed by a thermostat.] Fig. 3 is a slightly different arrangement of the same boiler as that shown in Fig. 2, connected with two radiators on different floors. The same riser supplies both radiators with steam and takes the water of condensation back to the boiler.