128 EXPEEIMENTAL GENERAL SCIENCE 



30. Why may a wind blowing across a chimney cause a strong upward 

 draft in it? 



31. At the mouth of large caves, a current of air flows outward or 

 inward at an approaching change of the weather. In which direction 

 do you infer the current of air will flow just before stormy weather? 

 Why? 



32. In correcting barometer readings, it is customary to add certain 

 amounts to the readings of all places above sea level. Why is this 

 necessary in comparing the pressure at different places? 



33. Why is it necessary to correct the barometer readings for tem- 

 perature? 



34. What is the total air pressure on the nearest book? Why can 

 you lift the book from the table? 



36. How does the varying air pressure ventilate the soil? 



