BAROMETER. 27 



84. There is no pressure upon the top of the mercury 

 in the tube, and the pressure of the air upon the mercury 

 in the bag forces up the mercury in the tube till its 

 weight exactly balances the weight, or downward pres 

 sure, of the air. The pressure of the air is sometimes 

 greater, sometimes less, but is commonly sufficient to 

 balance the downward pressure of a column of mercury 

 29 or 30 inches long. As the pressure of the air increases, 

 it causes the mercury to rise higher ; as it diminishes, it 

 allows the mercury to fall lower ; and these changes are 

 seen, by observing how high the surface stands as marked 

 on the graduated scale. Changes in the weather are 

 sometimes foreshown by changes in the height of the 

 mercury as indicated by this scale. 



85. The downward pressure, or weight, of a column 

 of mercury 30 inches long, and an inch square at the 

 bottom, is 15 pounds ; and as this column is sustained 

 by the pressure of the air, every where near the level of 

 the sea, we conclude that the pressure of the air, on 

 every square inch, is 15 pounds. 



86. When the mercury in the tube is slowly and gradu 

 ally rising, it commonly indicates the approach of fine 

 weather. &quot;When it is regularly and slowly falling, it 

 indicates foul weather. A rapid and sudden fall of the 

 mercury threatens a violent wind. 



While it is rising, the surface of the mercury is convex, 

 or swelling upwards ; when falling, concave, or hollowing. 



87. A very compact and convenient barometer is made 

 at Lowell, Mass., of a somewhat different construction. 

 A short column of mercury, in a glass tube, (c D fig. 1,) 

 is pressed upon, at the upper surface, by the atmosphere, 

 with which it has communication. The other end of the 



