The =hot water= is a very desirable system of heating. Heat convection through water is practically the same as that through air. The particles of water at the point where the heat is applied become lighter as they become heated and because of this change naturally rise to the top and the particles that are cooler and therefore heavier sink to the bottom, thus forming currents. For this reason water heats much more rapidly when the heat is applied to the bottom than it does when heat is applied at the top of the receptacle. These principles are utilized in hot water systems. The hot water rises in the radiators and gives off its heat to the cooler atmosphere and the cool water returns to the boiler to be reheated. In this way there is a constant circulation of the water through the system. The hot air system is entirely satisfactory for small buildings, but is not so desirable for large buildings. For large dwellings and public buildings the hot water system is much more satisfactory. In the hot water system a water jacket is provided instead of an air jacket. The water is carried from the boiler over the fire box through pipes to the different rooms where it passes through radiators and is returned to the boiler by continuous pipes. At the top of the pipes an expansion tank is placed to take care of the expansion of the water when heated. The heated water circulates freely through the system of pipes. Each radiator is provided with a valve to regulate the amount of water admitted and in this way the degree of heat may be regulated in the different rooms. Each radiator is provided with an air valve to allow the air to escape when the water is admitted, or to allow the steam to escape in case the system becomes overheated and the water is converted into steam. [Illustration: _Central Heating Showing boiler, pipes, and radiators._] This system is easily cared for and easily operated. It requires a relatively small amount of coal and maintains a very even heat, and when properly operated there will be no sudden changes in the temperature of the rooms. There is less danger of overheating the air and lowering the humidity with this system than with the hot air furnace. The radiators may be placed in the rooms that are to be heated or they may be placed in the basement and the air admitted and passed over the radiator and warmed and then forced into the rooms. This is known as the indirect method and is used in connection with the system of ventilation. It is used more with steam than with hot water. Heating by =steam= is by far the most satisfactory method, especially for larger dwellings and buildings. This system is somewhat like that of hot water, except the water is converted into steam. The temperature of the pipes and radiators is therefore higher than with hot water, but the pipes and radiators do not need to be as large.