Gravitation 13 



these spaces. The air pressure on the water is like your 

 foot on the mud, and the space in the lamp globe is 

 like the space between your toes. Since wherever 

 there is air it is pressing hard, the only space into which 

 it can force water or anything else is into a place from 

 which all the air has been removed, like the inside of 

 the lamp globe. 



The reason that the water does not run out of the 

 globe is this : the hole is too small to let the air squeeze 

 up past the water, and therefore no air can take the 

 place of the water that might otherwise run out. In 

 order to flow out, then, the water would have to leave 

 an empty space or vacuum behind it, and the air pres- 

 sure would not allow this. 



Why water gurgles when it pours out of a bottle. 

 You have often noticed that when you pour water out 

 of a bottle it gurgles and gulps instead of flowing out 

 evenly. The reason for this is that when a little water 

 gets out and leaves an empty space behind, the air 

 pushing against the water starts to force it back up ; 

 but since the mouth of the bottle is fairly wide, the 

 air itself squeezes past the water and bubbles up to 

 the top. 



Experiment 3. Put a straw or a piece of glass tube 

 down into a glass of water. Hold your finger tightly over 

 the upper end, and lift the tube out of the water. Notice 

 how the water stays in the tube. Now remove your finger 

 from the upper end. 



The air holds the water up in the tube because there 

 is no room for it to bubble up into the tube to take the 

 place of the water ; and the water, to flow out of the 



