302 SCIENCE OF HOME AND COMMUNITY 



kept the balloon in the air for about five minutes by throw- 

 ing straw and wood on the fire. 



About a month later another ascent was made by two 

 men, and the balloon was allowed to go free with the wind. 

 The men descended about five miles from the starting place, 

 the journey taking about 25 minutes. 



In less than a month after this, another ascent was made 

 by two other Frenchmen in a balloon filled with hydrogen 

 gas. The journey lasted about an hour and a half and the 

 men landed about thirty miles from the place from which 

 they started. The first ascent in America was made during 

 the last of December of the same year. After this, ex- 

 periments were carried on in other countries and many 

 improvements were made. 



Why a balloon rises. The principle involved in the rise 

 of the balloon is similar to that in the floating of a boat. 

 It is much the same as though a diver in the water should 

 set free a block of wood. It would at once rise to the surface 

 because it is lighter than water. The balloon rises till it 

 weighs the same as an equal volume of air around it, the air 

 being lighter the higher up one goes. The balloon con- 

 tinues to float at this height and is carried by the wind till 

 some of the gas escapes, and then the balloon gradually falls 

 to the ground. When the balloonist wishes to rise higher, he 

 throws out sand bags which have been placed in the basket 

 of the balloon ; this makes it lighter, and hence it rises. If 

 the balloonist wishes to descend, he pulls a line that opens a 

 valve which allows the gas to escape, and so the balloon 

 sinks. 



DEMONSTRATION 23 



Purpose. To illustrate the principle of the balloon. 



Apparatus. Large-mouth bottle, rubber stopper with two 

 holes, thistle tube, zinc, hydrochloric acid, rubber tubing. 



Directions. Put some granulated zinc in the bottle. Bend 

 a short glass tube at right angles and push one end in one of 





