152 



GENERAL SCIENCE 



continuously caught under the planes, causes the whole 

 machine to be lifted before the air has time to move from 

 under it. 



103. Barometers Kinds and Their Uses. There are 

 two kinds of barometers, the mercurial and the aneroid. 



U. S. GOVERNMENT EXPERTS TESTING A BALDWIN DIRIGIBLE AIRSHIP 



There are several types and modifications of each kind. 

 Barometers do not, as some think, foretell the weather 

 or measure the height of mountains. Barometers measure 

 only one thing, and that is the air pressure. But air pres- 

 sure and weather conditions have a very close relation. 

 By hourly readings of the barometer one can tell whether 

 the mercury is falling or rising and how fast. A falling 

 barometer indicates foul weather and a rising barometer 

 indicates the approach of fair weather. A comparatively 

 sudden drop of the mercury means that a storm is ap- 

 proaching. After the storm is past the mercury will 



