PRESSUKE OF THE AIR 



121 



water vapor and dust, acts as a blanket to keep 

 in the heat and prevent sudden changes in the 

 temperature of the air. 



106. The Barometer. Owing to various 

 causes, the pressure of the air in a given locality 

 varies from day to day. The most important of 

 these causes are differences in the temperature 

 of the air and varying amounts of water vapor in 

 it. Moist air is lighter than dry air, and warm 

 air lighter than cold. Variations in air pressure 

 are measured by the barometer. This instrument 

 consists of a glass tube closed at the upper end, 

 and filled with mercury. The lower end of the 

 tube dips into a dish of mercury and the pressure 

 of the air on the surface of the dish causes the 

 mercury in the barometer tube to rise and fall 

 with each variation in pressure. A scale is 

 attached to the upper end of the tube by means 

 of which the variations may be ascertained. 

 A sliding indicator and scale called a vernier is 

 attached to the tube for exact measurements. 

 At sea level the pressure of the air is sufficient 

 to support a column of mercury 760 millimeters 

 or 30 inches long. With each rise of 90 feet 

 above sea level the mercury lowers about Ho 

 inch. The daily fluctuations of the barometer 

 seldom vary more than a few tenths of an inch. 

 This, however, represents a considerable differ- 

 ence in pressure. A difference of one inch in 

 barometer readings indicates a difference of 

 more than a million tons over a square mile. FIG. 39. 

 Although mercury is the most satisfactory fluid barometer^ 

 for use in barometers, any other liquid might (Tower, Smith 



. ,. , . ,. ,, and Turton.') 



serve to indicate pressure, but in case a lighter 



