The most desirable system of heating is the central. In this system the heat is conveyed from the central heating plant to the rooms either by air, hot water, or steam. The heat may be produced in the house that is to be heated, usually in the basement, or it may be produced at a distance, as in the case of steam plants, and carried through pipes to the house. There are three principal systems of central heating: Hot air, hot water, and steam. Heat is carried through air by convection. The air exposed to heat becomes specifically lighter and hence rises and the cooler air takes its place. In this way the air of a room is heated by its constant movement brought about by this phenomenon. The air becomes heated in the air jacket of the hot air furnace and creates an upward draft. As soon as the cool air rushes downward to take its place a downward draft is formed through the cold air ducts. In this way the air in the room is kept in circulation and at the same time properly warmed. =Hot air=, or as it is sometimes called, furnace heating, is a very satisfactory system for an ordinary dwelling or small building. It consists of a large stove much as that used in local heating. Surrounding this stove is a jacket with an air space between it and the stove. Pipes lead from the air space through the top of this jacket and convey the air that is heated by the stove to the different rooms. The cold air is taken from the rooms and conveyed through the cold air pipes back to the furnace and is admitted to the air jacket from beneath. In this way the warm air, being lighter than the cold air, passes upward through the hot air pipes and is replaced by the cold air through the cold air pipes. In this way there is a constant circulation of air through the pipes. This kind of a furnace requires much attention, but not as much as a local heating stove. Great care must be taken that the air is not overheated in the air jacket. If it is, the air in the room will be dry and stuffy and may even have a scorched odor. The furnace should be equipped with a water receptacle inside the air jacket. This receptacle should be kept full and in this way a proper relative humidity will be maintained. If this is not done, the air in the room will be too dry and it will require a higher temperature to keep the room comfortable. Another objection to this system is that dust and dirt are likely to enter the rooms from the furnace.