PRESSURE OF THE AIR 125 



tinues to run. Siphons are used in emptying tubs, barrels, 

 cisterns, and in removing liquid from bottles, and the like, 

 without disturbing the sediment in the bottom, and for many 

 other purposes. 



Practical Exercises 



1. Press a drinking glass or test-tube, mouth down, into a vessel of 

 water. Why does the water not fill it? 



2. Why is a so-called empty bottle not really empty? 



3. Stop up one end of a glass tube with the finger, fill it with water 

 and invert it, placing the open end in a dish of water. Why does the 

 water not run out? 



4. Remove your finger from the end of the tube. Why does the water 

 now run out? 



5. Fill a drinking glass with water, cover with a card or piece of stiff 

 paper and, holding the paper with one hand, invert the glass. Why does 

 the water not run out when the hand is removed? 



6. If the glass in the above experiment were exactly an inch square 

 in cross-section, how many pounds of water could be supported in this 

 way? 



7. Suppose the experiment in exercise 5 had been performed with 

 mercury instead of water. Would the pressure have held up as great a 

 volume of mercury? Why? 



8. Examine the nearest barometer and find out how high a column 

 of mercury the air is able to support at the time the observation is 

 made. 



9. If it is less than 760 millimeters, account for the difference noted 

 (104). 



