ATMOSPHERIC PRESSURE 



55 



home with a football or a basket ball. Suspend a yard- 

 stick at its middle point. Hang a fully inflated football 

 from one arm about twelve inches from the center. The 

 rubber tube should extend and be closed with a clamp. 

 Balance the ball exactly with weights on the other arm. 

 Open the clamp to allow the excess of air to escape. The 

 ball will rise, showing that it is lighter and has lost weight. 

 If it has lost air and weight, what is your conclusion about 

 whether air has weight or not ? 



Atmospheric Pressure. Since air has weight, it must 

 exert force upon the objects upon 

 which it rests like all other matter. 

 Torricelli (tor're-chel'le), a pupil 

 of that great Italian scientist, Gali- 

 leo, proved that air exerts pressure 

 by means of the following experi- 

 ment. 



He took a glass tube about three 

 feet long, closed at one end, and 

 filled it with mercury. Then hold-* 

 ing his thumb over the end, he in- 

 verted it in a cup of mercury. The 

 column of mercury dropped until 

 the height was about thirty inches 

 above the mercury in the cup. This 

 showed that the pressure of air on 

 the mercury in the cup was sufficient 

 to balance a thirty-inch column of 

 mercury. Torricelli called the in- 

 strument he used for measuring air 

 pressure a barometer. Later, when why does the column of 



the barometer Was Carried to the top mercury remain at a height 

 . . . , , , T i / of 30 inches ? 



of a mountain three thousand feet 



high, the mercury column dropped about three inches. 



Can you explain why? 



