DEVELOPMENT OF HOUSE HEATING 121 



pours upward against the ceiling, it spreads out in every 

 direction toward the outside walls. The outside walls, and 

 especially the windows, are cold, consequently the air is here 

 chilled. It therefore becomes heavy and drops to the floor. 

 Across the floor from every side of the room the cold stream 

 of air passes back to the stove. As a person sits facing the 

 stove in such a room there is sure to be a stream of cold air 

 blowing against his back and past his feet. This is especially 

 true if the stove is not large enough to heat all the air in 

 the room. These returning currents of air are often mis- 

 taken for cold, outside air which is supposed to have crept 

 in through cracks and crevices about the windows and in 

 the outside wall. If the walls of the room were air-tight 

 these currents would still exist. 



139. Radiant Heat. Not all of the heat from the stove, 

 however, is distributed through the room by means of these 

 convection currents. Often from one-fourth to one-third is 

 given off in what is called RADIANT HEAT. 



Exercise 38. Radiant Heat 



Heat a poker red hot. While holding it with one hand, place the 

 other hand about 2 in. above it. Can you feel the heat? Hold 

 your hand about 2 in. from its side. Can you still feel the heat from 

 it? Is the heat carried by the convection current? Place the hand 

 about 2 in. below the poker. Can you still feel the heat? Certainly 

 this heat is not carried by convection currents. 



This experiment shows clearly that some heat is sent off 

 from the poker to each side and downward, as well as up- 

 ward. This heat is said to be RADIANT HEAT, and it is said 

 to have passed from the poker by RADIATION. While it is 

 difficult to explain radiation and radiant heat, it is very easy 

 to show some of the results of radiation of heat from the 

 ordinary stove. Before we study these applications we 

 shall state the LAWS OF RADIATION as they have been deter- 

 mined by careful experiments: 



1. Heat is radiated from a heated body in straight lines in 

 all directions and in equal amounts in every direction. 



