THE USE OF WEATHER INSTRUMENTS 179 



The class should continue these daily observations and the 

 record of them until ready to use weather instruments. 

 (Art, 227). 



The weather record should be carefully studied once each 

 week and answers written to the following questions if possible : 



1 . Does the sky appear to be more or less cloudy for the few hours 

 preceding a storm? How is it for the few hours after the storm? 



2. Is there any direction from which the wind appears to blow 

 most frequently before a storm? If so, what direction? Is there any 

 direction from which it blows most frequently after a storm? 



3. Does the wind increase, or decrease, in the few hours before a 

 storm? Does it blow harder before, or after, a storm? 



4. Does the temperature seem warmer, or colder, before a storm? 

 How does it change after a storm, if at all? 



5. Does the "feeling " of the air ever tell you that a storm is coming? 

 If so, can you tell what it is about the air that makes you think a 

 storm is to come? How long before the storm did you notice that 

 "feeling" of the air? 



II. THE USE OF WEATHER INSTRUMENTS 



The most important weather instrument is the barometer, 

 which measures the pressure of the air. 



208. Air has Weight. When compared with any common 

 liquid or solid, air is so light that its weight is not easily 

 recognized. Yet smoke rises through the air, and balloons 

 ascend out of sight. This is possible only because air is 

 heavier, or denser, than the smoke or the gas in the balloon 

 and crowds them upward, just as a cork rises to the surface 

 of water because water is heavier, or denser, than cork and 

 crowds it upward. But we can not see the air; we scarcely 

 feel it; we seldom think of it. That such a substance con- 

 stantly surrounds us, filling every nook and corner of our 

 houses, even penetrating deep into the ground itself; that it is 

 constantly pressing with great force upon our bodies; these 

 things many of us have not realized. 



It was not until 250 or 300 years ago that even men of 

 science began to understand that air is a real substance and 



