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 A radiator gives out its heat to its surroundings by radiation to the walls and objects and by convection to the air. What proportion is given out in each way it is difficult to measure in any case and depends principally on the construction of the radiator and the way it is set, but also more or less upon the conditions of temperature, nature of surface, etc. Péclet, the great French physicist, in the middle of the century, fully investigated the laws of radiant heat as well as those of convection in still air. The formulas which he deduced are applicable only in a limited degree to radiator practice. His investigations, as well as those of others since that time, showed that for a single iron pipe in still air under conditions of temperature which prevail in radiator practice, the heat given off as radiant heat is just about equal to that given out by convection.

But radiators are invariably built of clusters of pipes or surfaces, and as radiant heat travels only in straight lines and perpendicular to the surface of its source, a large proportion of the surface is wasted so far as radiant heat is concerned, due to what may be called the mutual interception of the rays. In the ordinary one-column cast-iron radiator, the proportion of surface from which no radiant heat takes place is nearly 20 per cent., in the two-column, 45 to 55 per cent. and in the three-column, 55 to 65. Assuming that the radiant heat amounts to one-half the total, the reduction of heat emitted would be one-half of these percentages. Another fact which further reduces the radiant heat is that radiators are usually set very close to a wall which becomes heated to a comparatively high degree and consequently radiates back a large portion of the heat to the wall side of the radiator. This is true to an extreme degree in the case of indirect radiators which are enclosed in boxes of wood or sheet metal and are not located in the room they are to warm. With these the heating is accomplished entirely by convection, while with direct radiators the radiant heat rarely amounts to 40 per cent., generally not over 30, and often in practice considerably less. For a radiator in any particular location the radiant heat is constant for the same conditions