118 THE PRODUCTION AND USE OF HEAT 



the bottom of the atmosphere exactly as it fell through the 

 water and rested at the bottom of the pail. Even the cork is 

 so much heavier than the air that it, too, sinks to the bottom 

 of the atmosphere. There are some substances lighter than the 

 air. Hydrogen weighs but about one-fifteenth as much as the 

 air. A BALLOON is a bag filled with this very light hydrogen 

 or some other light gas. Whenever the bag and the hydrogen 

 which it contains weigh less than the air it displaces, the 

 surrounding air buoys it up with a force greater than its weight 

 and it floats. Instead of using hydrogen, heated air is often 

 used in toy balloons. The toy Fourth of July balloon con- 

 sists merely of a sack of light, nearly air-tight 

 material, usually paper. The bag is inverted and 

 its lower end is somewhat open (Fig. 89). A 

 burning candle is suspended in the open mouth 

 of the sack. The heat from the candle keeps the 

 FIG. 89. air in the baUoon warm. As long as this air is 

 Hot-air sufficiently heated, the balloon continues to float. 

 The important thing to notice is this: The heated 

 air would be buoyed up, pushed upward, just the same if it were 

 not enclosed in a sack. From this fact we see that convection 

 currents are always produced when any fluid is heated more at 

 one point than at surrounding points. 



We shall find convection currents of great importance in 

 the study of the weather, Chap. IV. 



APPLICATION OF CONVECTION CURRENTS TO CHIMNEYS 



136. The Draft in the Chimney. The current of air, or 

 draft, in the chimney is caused by the column of air within 

 the chimney becoming either warmer or cooler than the 

 surrounding air. If the air within the chimney is warmer, the 

 draft will be upward. Why? When there is a fire in the 

 furnace or stove, the air within the chimney will be heated 

 and the draft will be upward. It is now evident that the 

 draft is not strong when we first light a fire in the stove be- 

 cause the air within the chimney has not yet been heated. 



