CURRENTS IN AIR AND WATER 89 



vividl; illustrated by a miniature room formed of a soap box into 

 the sides of which small windows are cut. Short pieces of candles 

 placed on the bottom may be used as the sources of heat. The 

 top of the box is covered with a board, or better, if one wishes to 

 watch the condition of the candles, with a sheet of glass. The 

 windows are closed by pasting paper over them and opened by 

 . tearing it away. By opening and closing the windows in diflerent 

 combinations it can be seen under what arrangement the candles 

 burn best. Good ventilation keeps them bright; poor, dims them 

 or puts them out. The smoke paper shows into which windows 

 the air enters and from which it escapes. Some of the pupils may 

 make a more elaborate miniature house into which ventilating 

 flues may be introduced according to some system used which 

 they may have seen placed in some building during its construc- 

 tion. 



To shov/ how a heated mass of air may reach a considerable 

 height, a balloon made of tissue paper filled with hot air serves 

 excellently. In the hot air balloon we have, of course, a defin- 

 itely outlined mass of heated air whose movement and course we 

 may actually watch. Such a balloon can often be purchased at 

 the toy counter, but one made by the school children has more of 

 intere.'Jt in it. One not less than tour feet in diameter will give 

 the greatest satisfaction. The mouth must be pretty wide or it 

 will catch fire in its swaying about. In the school room the bal- 

 loon may be filled with heated air by holding it over a large lamp. 

 When thus filled it will rise slowly to the ceiling and when cooled 

 will gridually sink to the floor. 



Out of doors the heat for filling the balloon is best made by 

 building a fire at the bottom ol a joint of stovepipe fixed in an 

 upright position. This will be tall enough to prevent the flames 

 catching the balloon. By this means sufficient heat is furnished 

 to carry the balloon up but a moderate distance. To send it up a 

 good distance a piece of cotton soaked with paraffin suspended 



