Mingling of Molecules 



283 



Clear weather, on the other hand, is often the result 

 of air being compressed, for compressing air warms it. 

 When air is being warmed, the water vapor in it will not 

 condense ; so the air remains clear. But when the air is 

 being compressed, it presses hard on the mercury of the 

 barometer ; the pressure is high, and the mercury in the 

 barometer rises high. Therefore when the mercury in the 

 barometer is rising, the weather is usually clear. 



These two statements are true only in a very general 

 way, however. If weather forecasters had only their 

 own barometers to go 

 by, they would not be 

 of much value; for one 

 thing,. they could not tell 

 us that a storm was 

 coming much before it 

 reached us. But there 

 are weather stations all 

 over the civilized world, 

 and they keep in touch 

 with each other by tele- 

 graph. It is known that 

 storms travel from west 

 to east in our part of the 

 world. If one weather 

 man reports a storm at 

 his station, and tells how 

 his barometer stands, the 

 weather men to the east 

 of him know that the 

 storm is coming their 



FIG. 154. Finding the pressure of the air 

 by measuring the height of the mercury in 

 the tube. 



