METHODS OF HEATING 349 



through cracks at windows and doors. This loss of heat 

 outwards and cold air leakage inwards increases as the differ- 

 ence between the temperature of the inside and outside air in- 

 creases. Double-windows, storm-doors, building-paper under 

 shingles, clapboards, and plastering tend to check these losses. 



392. Radiators and Radiation. Radiators are made up of 

 hollow sections of cast iron. The outer surfaces are so shaped 

 as to give the greatest possible area or, as it is generally 

 called, the greatest radiating surface. The castings for 

 radiators are purposely made rough, and are often elaborately 

 figured in pleasing designs, so as to present a larger radiating 

 surface than would be the case if they were smoothly finished. 

 The transfer of heat from hot metal surfaces to air is more 

 efficient if the radiating surface is rough and the color is dark. 

 Radiators are sometimes gilded for appearance, but practi- 

 cally they do not heat as well as if left ungilded. A cheap form 

 of radiator for stores and shops is cast with innumerable pro- 

 jecting plugs or pins. 



For large rooms a radiator may be made up of pieces of 1 in. 

 or 1)4 m - pip e joined together by elbows and return bends. 

 Such a radiator is called a box coil. A more common method 

 of installing a direct radiation system is to run a group of 

 l^in. steam pipes along the side of a room and around the 

 corner, by means of couplings to provide for expan- 

 sion and contraction, and to connect the ends of the run 

 into branch trees. The advantage of this arrangement is 

 that it distributes the heat throughout the whole length of 

 the room. 



In a dwelling house the radiators are generally placed near 

 the windows, since the cold air then reaches the radiators 

 quickly. The direction of the flow of air -along the floor is 



