In the systems of steam heating which have been described up to this point the pressure carried has always been above that of the atmosphere, and the action of gravity has been depended upon to carry the water of condensation back to the boiler or receiver; the air in the radiators has been forced out through air valves by the pressure of steam back of it. Methods will now be taken up in which the pressure in the heating system is less than the atmosphere and where the circulation through the radiators is produced by suction rather than by pressure. Systems of this kind have several advantages over the ordinary methods of circulation under pressure. First - no back pressure is produced at the engines when used in connection with exhaust steam, but rather there will be a reduction of pressure due to the partial vacuum existing in the radiators; second - a complete removal of air from the coils and radiators so that all portions are steam filled and available for heating purposes; third - complete drainage through the returns, especially those having long horizontal runs, and the absence of water hammer; and fourth the smaller size of return pipes necessary. The two systems of this kind in most common use- are known as the Webster and Paul systems.