PRESSURE OF THE AIR 127 



21. What effect would moisture in the air be likely to have on the 

 height of the mercury in the barometer? 



22. Which do you infer that a falling barometer indicates, fair or 

 stormy weather? 



23. Why does smoke fail to rise well from chimneys just before a 

 storm? 



24. Why may the stove fail to have a good draft on a rainy day? 



25. Spread out one hand, palm downward, and hold a piece of paper 

 about two inches square up against the first and middle fingers where 

 they join the hand. Blow downward between the fingers and the 

 paper will remain in position without being held, as long as one blows. 

 The breath makes a partial vacuum on the upper side of the paper. 

 Explain the pressure that holds the paper in place when it is being blown 

 upon. 



26. In the vacuum cleaner, a rapidly revolving fan, consisting of a 

 wheel with curved blades, makes a partial vacuum in the interior of the 

 machine. Explain >* >w this causes the dirt to be swept into the cleaner. 



27. The system of pipes that carry away the dust from wood-working 

 machinery, emery wheels, and the like, is essentially part of a huge 

 vacuum cleaner. A similar system of pipes is often used in ventilating 

 mines and large buildings, the impure air being pumped out by this 

 means. Explain how this causes the fresh air to enter. 



28. The electric fan works on the principle of the vacuum cleaner, 

 though in this case, it is the outflowing currents of air, instead of those 

 flowing in, that we value. Where is the partial vacuum that is produced 

 by the fan located? 



29. Where is the partial vacuum in an atomizer? 



