282 



Common Science 



into a vacuum, the mer- 

 cury in the tube would 

 sink down into the cup. 

 But the pressure of the 

 air on the surface of the 

 mercury in the cup keeps 

 the mercury from flow- 

 ing out of the tube and 

 so leaving a vacuum in 

 there. If the air pushes 

 down hard on the mer- 

 cury in the cup, the 

 mercury will stand high 

 in the tube. This is 

 called high pressure. If 

 the air does not press 

 FIG. 153. inverting the filled tube in the hard on the mercury in 



cup of mercury. ^ ^ ^ mercury 



stands low in the tube. This is called low pressure. 



How weather is forecast. Weather forecasters make 

 a great deal of use of the barometer, for storms are 

 usually accompanied by low pressure, and clear weather 

 nearly always goes with high pressure. 



The reason storms are usually accompanied by low 

 pressure is this : A storm is almost always due to the 

 rising of air, for the rising air expands and cools, and if 

 there is much water vapor in it, this condenses when it 

 cools and forms clouds and rain. Now air rises only 

 when there is comparatively little pressure from above. 

 Therefore, before and during a storm there is not so much 

 pressure on the mercury of the barometer and the ba- 

 rometer is low. 



