THE AIR 



153 



rapidly rise again. By keeping a record of your readings 

 of the barometer and your observations of the weather, 

 you can learn to foretell the weather several hours or 

 even a day or two in advance. 



Since the density of the air decreases as one ascends a 

 mountain, the height of the mountain can be determined 

 by measuring, with a barometer, the air pressure at its 

 base and on its summit. The mercury falls 

 about one inch for every 900 feet of vertical 

 ascent. If the mercury reading is five inches 

 less at the top of a mountain than the reading 

 at its base, how high is the mountain? The 

 height of buildings and hills can also be de- 

 termined by taking the readings of the ba- 

 rometer at the bottom and at the top. The 

 height of mountains as generally given indi- 

 cates the height of the summit above sea 

 level. How is this found? 



(a) The Mercurial Barometer is essentially the 

 same as the simple barometer tube described 

 in 102. It was first used by Torricelli in 

 1643. The modern barometer has additional 

 fixtures to prevent the mercury from spilling 

 out of the cup or cistern and for accuracy in 

 reading the height of the mercury column. This 

 height varies from 29.2 to 30.5 inches, or from 

 73 to 76.5 cm., in localities which are not far 

 above sea level. The reason for these changes A MERCU . 

 in height is that disturbances of the atmos- 

 phere affect the air pressure at the earth's sur- 

 face. Mercurial barometers can be made the most accu- 

 rately, and are used in the United States Weather Bureau 

 offices when careful observations are made. 



RIAL BA- 

 ROMETER 



