3 8o 



AIR 



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 30 inches high. 



The barometer. Since the pressure of the air 

 changes from time to time, the height of the mer- 

 cury changes also. When the air pressure is heavy, 

 the mercury is high ; when the air pressure is low, 

 the mercury shows a shorter column. By reading 

 the level of the mercury one can learn the pressure 

 of the atmosphere. If a glass tube and a dish of 

 mercury are attached to a board and the dish of 

 mercury is inclosed in a case for protection from 

 moisture and dirt, and if a scale of inches is made 

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

 FIG. 249. A curial barometer (Fig. 249). 



simple ba- 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 of the air 

 decreases. Observations similar to 

 these were made by Torricelli as early 

 as the seventeenth century. Taking a 

 barometric reading consists in measur- 

 ing the height of the mercury column 

 supported by the atmosphere. 



A portable barometer. The mer- 

 cury barometer is large and inconven- 

 ient to carry from place to place, and 

 a more portable form has been devised, 

 known as the aneroid barometer (Fig. 

 250). This form of barometer is ex- 

 tremely sensitive; indeed, it is so delicate that it shows the 

 slight difference between the pressure at a table top and the 



FIG. 250. Aneroid barometer. 



