The usual temperature to which the small house is heated when it is zero outside is 70 degrees Fahrenheit. It is then assumed that a certain quantity of heat is lost through the walls of the house by radiation and convection and conduction, and another quantity lost by the leakage of warm air out through the window-cracks. (The quantity of heat is measured in British thermal units, called B. T. U.’s.) To understand the manner by which heat is lost through the exterior walls, it is necessary to know the meaning of radiation, convection, and conduction. By standing before an open fire the heat given off by radiation can be observed by shutting it off with a piece of paper held between the face and the fire. This is the transmission of the heat through the ether, and is similar to the transmission of light, since this heat will pass through glass, like light. Convection of heat is illustrated by heating air in one place and transferring that air to another place, where it will give up its heat to surrounding bodies. Conduction of heat is illustrated by heating the end of an iron rod and noticing that the heat will eventually be transmitted along the length of it to the other end. The heat within a house escapes from the interior to the colder atmosphere of the exterior through the walls, by radiation through the glass windows and the substance of the walls, by the convection action of the warm air of the interior giving up its heat to the interior face of the wall and the cold air of the exterior extracting this heat from the exterior face and carrying it off, and also by the action of conduction of the materials of which the wall is composed. The quantity of heat lost is measured by the number of B. T. U.’s lost through one square foot of the wall each hour. As the window-glass loses heat through it more quickly than the wall, it is necessary to calculate this separately. The process, then, for estimating the heat loss from a room is as follows: 1. Estimate the number of square feet of exposed wall surface in the room, including windows. 2. Subtract from the above the area of the windows to find the net wall area. 3. Multiply this net wall area by the number of B. T. U.’s which the wall loses per square foot of surface for each hour. These factors are given in the following table: Zero outside and 70 degrees inside—Number of B. T. U.’s TYPE OF WALL lost for each square foot of wall surface each hour Brick wall, furred and plastered: 8" thick 21.0 12" thick 17.5 Frame wall, sheathed, clapboarded, and plastered 21.7 (with building-paper use 20.3)