PROPERTIES OF GASES 



97 



at that height it exerts very little pressure. About half the 



weight of all the air is within three miles of the earth. This 



is because the lower part of the atmosphere, having the 



weight of all the upper air 



resting upon it, is compressed 



and therefore has greater 



density. 



105. The Barometer. - 

 An instrument for measuring 

 the pressure of the air is 

 called a barometer. In a 

 simple form, it consists of a 

 glass tube about three feet 

 long closed at one end; the 

 tube contains mercury and 

 stands inverted m a cup of 

 mercury. The mercury in 

 the tube stands at the height 

 of about thirty inches above 

 the level of the mercury in 

 the cup. 



As the tube is open at 

 the bottom, the mercury 

 is free to leave it, but does 



not do SO because the air 

 is pressing down Upon the 

 mercury in the CUp. The 



pressure is transmitted 



, 1 



through the liquid both height of only 30 in.? 3. Suppose that 



horizontally and upwards, both ends were open; why would the 



mercury not remain in the tube? 



and thus the downward 



pressure of the air holds the column of mercury in the 



tube. 



We know that the weight of thirty inches of mercury in 

 a tube one square inch in area is nearly fifteen pounds. The 



FIG. 45. TORRICELLI'S 

 EXPERIMENT 



In 1643 an Italian named Tor- 

 ricelli observed, in using apparatus 

 like this, that mercury would remain 

 in a closed tube to the height of 30 in. 

 if the tube were inverted with the 

 open end in mercury. 1. He dis- 

 covered the reason. What is it? 

 2. Why will the mercury remain at the 



