THE USE OF WEATHER INSTRUMENTS 



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The corrected barometer readings everywhere in the United 

 States will be exactly alike unless there are differences in the 

 actual pressure of the atmosphere over different regions. 

 Differences in atmospheric pressure are not noticeable as are 

 differences in temperature or cloudiness or wind. They could 

 not be known if we had no barometers. But these differences 

 in pressure cause most of the weather changes that occur. 

 And when the corrected barometer readings differ, they 

 show on the weather map where storms or fair weather areas 

 are, and so enable the Weather Bureau to make forecasts of 

 coming weather. Barometers, used in that way, are there- 

 fore the most important of weather instruments. 



PROBLEMS 



Find the corrected barometer reading from the following data: 



216. The Barograph. In the barograph, the record paper 

 is wrapped around a brass cylinder that is turned by an eight- 

 day clock. The barometer portion consists of a series of six 

 or eight hollow elastic shells shaped like a canteen or two 

 saucers turned top to top (Fig. 135). These shells are made 

 of corrugated metal and the air has been exhausted from 

 them. They are soldered together one above the other. It 

 is easily seen that the atmospheric pressure tends to crush 



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