CHAPTER II 

 FLUID PRESSURE 



SECTION I 

 PRESSURE IN LIQUIDS 



32. Fluids. Liquids and gases are called fluids 

 because they will flow. This property of liquids and 

 gases is due to the fact that their molecules move freely 

 among each other from place to place. The molecules of 

 a solid, of course, vibrate, each in its position ( 11), but 

 none of them can easily change its position among the 

 others ; hence a solid body preserves its shape. The 

 molecules of fluids, on the other hand, change their posi- 

 tion so easily that the simple force of gravity is enough 

 to move them, pulling each downward as far as it will go. 



Experiment 20. Pour a tumbler of water into a large flat dish ; 

 it spreads out to the edges of the dish. Pour it upon a larger sur- 

 face (a board or piece of glass) and note what happens. How 

 may this be explained ? What force acts upon the liquid ? Is its 

 action strongly resisted? 



Thus we learn why it is that liquids flow downward. 

 Gravity acts in the same way upon the molecules of 

 solid bodies also, pulling each of them downward; but 

 in them the force which holds each molecule in its place 

 among the others is greater than the force of gravity 

 upon it, so that it does not move. In liquids and gases 

 the molecules are free to move as gravity pulls them. 



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