Fire and Heat 103 



in the breathing space? How is this secured? Often one may find 

 a layer of dead air, cold and ill-smelling, resting upon the floor and 

 reaching possibly to the height of three feet or more. Hot air, when 

 admitted through vents a few feet below the ceiling, remains at about 

 the same level, and when the windows are open at the top, it escapes. 

 If the windows are opened at the bottom, the cold, fresh air sinks to 

 the floor. Warm and cold air should diffuse or mix. How can this 

 be brought about? 



The humidity of the air of a room may be determined by the 

 aid of the wet- and dry-bulb thermometers (page 46). Or- 

 dinarily the effect upon the mucous membrane of the nose and 

 throat is fairly indicative of the humidity. If a hot-air furnace 

 is utilized for heating, is it equipped with a water pan for making 

 humid the heated air ? What other provisions do you know 

 for supplying the air with sufficient moisture? 



What should be the temperature of a room in which physical 

 work is performed? Mental work? How is the temperature 

 of the room or building regulated, that is, does it depend upon 

 human control or a mechanical device? The instrument in 

 common use for this purpose is the thermostat. This depends 

 upon the expansion and contraction of metals under the in- 

 fluence of heat and cold, and is so adjusted that a variation of 

 about two degrees will cause the heat supply to be turned on or 

 shut off. 



43. Radiation. Heat from range or fireplace is transmitted 

 to objects in a different way from the two which have been 

 studied. As you sit in front of the fireplace your face and 

 hands may almost blister with the heat, and your back be 

 freezing. The air between you and the fire is decidedly cooler 

 than your face and hands. How can you verify this ? Objects 

 below the level of the fire and sufficiently near to it are warmed 

 and sometimes burned by the excessive heat. But a screen 

 placed between the fire and the objects cuts off the heat supply, 

 as you know, for you sometimes protect your face by means 

 of a paper or book. If the air were moving from the fire toward 

 you, it might bring heat to you by convection ; but convec- 



