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What is a barometer? If we should take 

 a glass tube closed at one end, the area of 

 the cross section of which is one inch square, 

 and fill it with mercury, and while thus filled 

 plunge the open end into a vessel of mercury, 

 it will be found that the amount of mercury 

 remaining in the tube above the level of the 

 mercury in the vessel will weigh about fifteen 

 pounds, if the experiment has been per- 

 formed at sea-level. This will vary, however, 

 according to the temperature of the air. Of 

 course barometers are tested when the air is at 

 a certain temperature. If the weight of mer- 

 cury in the tube is fifteen pounds, since it is 

 sustained by the air pressing down on the 

 mercury in the open vessel, it shows that the 

 air-pressure on that open vessel is equal to 

 fifteen pounds to the square inch. In prac- 

 tice, of course, the tubes are made very much 

 smaller. If the air changes so that it is 

 lighter than normal the mercury will fall in 

 the tube, because the pressure on the mercury 

 in the open vessel is less than fifteen pounds 

 to the square inch. And, again, conditions 

 may arise that will condense the air and make 

 it for the time being weigh more than fifteen 

 pounds to the square inch, in which case the 

 mercury will rise in the tube. Thus it will be 

 seen that the barometer will register the 

 slightest change in air pressure. 



Let us dwell for a moment on the causes of 



