84 AIR 



space in the top of the tube. If we measure the column in 

 the tube, we find its height is about one thirteenth of 32 feet 

 or 29 inches, exactly what we should expect. Since there is 

 no air pressure within the tube, the atmospheric pressure on 

 the mercury in the dish is balanced solely by the mercury 

 within the tube, that is, by a column of mercury 29 inches high. 

 The shortness of the mercury column as compared with that 

 of water makes the mercury more convenient for both experi- 

 mental and practical purposes. (See Laboratory Manual.) 



78. The Barometer. Since the pressure of the air changes 

 from time to time, the height of the mercury will change from 

 day to day, and hour to hour. When the air pressure is heavy, 

 the mercury will tend to be high ; when the air pressure is low, 

 the mercury will show a shorter column ; and by reading the 

 level of the mercury one can learn the pressure of the atmos- 

 -phere. If a glass tube and dish of mercury are at- 

 tached to a board and the dish of mercury is inclosed 

 in a case for protection from moisture and dirt, and 

 further if a scale of inches or centimeters is made 

 on the upper portion of the board, we have a mercu- 

 rial barometer (Fig. 44). 



If the barometer is taken to the mountain top, 

 the column of mercury falls gradually during the 

 ascent, showing that as one ascends, the pressure 

 decreases in agreement with the statement in Section 

 76. Observations similar to these were made by 

 Torricelli as early as the sixteenth century. Taking 

 a barometric reading consists in measuring the 

 F IG 4 _A ne ight o f the mercury column. 



simple ba- 79. A Portable Barometer. The mercury ba- 

 rometer. . , . . r 



rometer is large and inconvenient to carry trom 



place to place, and a more portable form has been devised, 

 known as the aneroid barometer (Fig. 45). This form of 



