FIRE AND ITS USES 155 



the house, there condenses to water again as it cools, which flows 

 back to the heater through return pipes. 



Since dry air is a poor conductor of heat it is important to 

 keep the air in the house moist; otherwise the heat from the 

 radiator does not readily pass to your body. It is quite as 

 important to have a hygrometer in the living-room to see that 

 the air is moist as it is to have a thermometer to see that the 

 temperature is correct. One feels comfortable when in fairly 

 moist air at 68 F., whereas in dry air the temperature may have 

 to be 75 to give the same feeling of comfort. Evidently it is 

 good economy to keep the air moist. This may be accomplished 

 by a water pan kept well filled in the hot-air furnace or by pans 

 of water hung on the radiators in hot-water or steam-heating 

 systems. 



Just as the fire in the fireplace or stove causes the heated air 

 to rise in the chimney because the heavier cool air forces it up, 

 causing a draft, so any mass of heated air surrounded by cooler 

 air rises as the cooler air pushes it up and comes in with a rush 

 as it takes its place. A great fire in the open heats the air 

 above it, and the surrounding cool air blows in as the hot air 

 rises, causing local winds (Fig. 61). When the air becomes 

 heated over any area on the earth as over a desert, it rises and the 

 cool air around it blows in. The equatorial regions of the earth 

 are hot, and the air over them rises. We do not notice rising 

 or falling air as a wind only air that is moving horizontally 

 along the surface of the earth. So in the equatorial regions there 

 is a belt of calms. The cooler air flowing in from north and 

 south along the surface of the earth on the edges of this belt of 

 calms makes winds. These do not blow straight from the 

 north or south, for the air is coming from regions where it is 

 rotating with the earth at less speed than that of the equatorial 

 region. Because of inertia the inflowing air tends to keep its 

 slower rate of rotation, and the more rapidly moving equatorial 

 region slips along under it from west to east, so that the winds 

 s_eem to come from the northeast north of the equatorial belt of 



