90 



APPLIED SCIENCE 



The air becomes thinner or rarified, the higher one goes and 

 the pressure becomes less and less on the mercury in the open 

 cup, so that the mercury in the long tube is made to fall. 

 If the distance the mercury will fall for every 100 ft. of alti- 

 tude is known, the height of a mountain may easily be 

 ascertained by noticing the height of the mercury column 

 first at the bottom of the mountain and then at the top. 



90. Aneroid Barometer. The barometer most commonly 

 made for commercial purposes is the aneroid barometer 



(Fig. 54). The word " aneroid" 

 comes from the Greek and means 

 "not wet," and was selected 

 because this type of barometer 

 operates without any fluids. It 

 consists of a round, metallic, air- 

 tight vacuum case, somewhat like 

 a watch, the lid of which, held by 

 metallic springs inside, rises and 

 falls with the pressure of the at- 

 mosphere. By means of levers 

 and a delicate chain inside, this 

 rise and fall is made to turn the 

 pointers on the index. The deflection may then be read 

 on the circular scale. 



FIG. 54. Aneroid Barometer. 



91. Properties of Air. The air or atmosphere which 

 surrounds the earth is a mixture of two very different gases 

 called oxygen and nitrogen. To every 21 parts of oxygen 

 the air contains 79 parts of nitrogen. There are always pres- 

 ent in the air some dust, moisture, and other impurities 

 when atmosphere is put in motion by the unequal distribution 



