There are three methods of heating; radiation, conduction and convection. There are two systems by which these methods are used: they are local and central. In local systems the heat is produced in the room by combustion or burning of fuel in grates and stoves. In central heating the heat is produced at a central place outside of the rooms and conveyed to them by hot air, hot water or steam. Even though these three methods of heating are usually given, it is difficult to draw hard and fast lines of demarcation between the different methods, for, as a matter of fact, they overlap to quite an extent. The element of radiation is involved in both conduction and convection. LOCAL HEATING Radiation The vibrating molecules of a heated substance will set into motion the ether of space and in this way the heat may be transmitted as wave motion. We have an illustration of this in the transmission of heat to the earth from the sun. Ether waves are generated by the violent vibration of the molecules of the sun and the vibrations are transmitted to the earth and they in turn generate molecular vibrations of the bodies of the earth. This is spoken of as radiant heat and is illustrated by the heat from the open fireplace. Open fireplaces give off heat by direct radiation. This is the oldest method of heating and has been in use for many generations. It is not a satisfactory method, however. The radiation of heat takes place through air very readily, but air is not a good conductor of heat. Heat may be radiated from the body very rapidly through the air to cold objects. As for instance, sitting near a cold wall one will feel chilly due to the radiation of the heat from the body to the cold wall, although the air in the room may be sufficiently warm to be otherwise comfortable. Heat may be readily radiated from an open fire, but it must be remembered that the intensity of the radiated heat is inversely proportional to the distance of the heated object from the place where the heat is produced. To illustrate: If one object is one foot from the source of heat, the open fireplace, and another object is three feet from the fireplace, the object that is farther away will receive only one-ninth as much heat as the one nearer. This is one of the disadvantages of the open fireplace as a means of heating a room. A fireplace is very cheery and gives a room a comfortable appearance and is very popular in the modern home. It is adequate to take the chill away when the weather is not very cold, but it is certainly a very undesirable means of heating a house in cold weather. [Illustration: _Local Heating_] Another objection to the open fireplace is that it requires a great deal of fuel. About 75% of the heat is lost through the chimney. There is, however, an advantage in the open fireplace since it affords an excellent means for ventilation; there is always a draft up the chimney. Conduction