PRESSURE OF THE ATMOSPHERE 



115 



FIGURE 61 



FIGURE 62 



It cannot be done as easily as before the air was exhausted. 

 Why? 



Experiment 46. Fill a glass tumbler even full of water and 

 press upon it a piece of writing paper. (Figure 61.) Be sure that 

 the paper fits smoothly to the rim of the tumbler. 

 Take the tumbler by its base and carefully invert 

 it over a pan. Does the water fall out ? If not, 

 why not? While the tumbler is in the inverted 

 position, insert the point of a pencil between the 

 paper and the rim of the tumbler. What happens ? 

 Experiment 47. Fill a bottle with clean water 



and fit it tightly with a rubber stopper having two 

 holes in it. Plug one of the holes tightly with a glass 

 tube one end of which has been closed by heating in 

 a Bunsen burner. Through the other hole put an 

 open glass tube 10 to 15 cm. long. See that both 

 tubes fit tightly in the stopper and that the stopper 

 fits tightly in the bottle. (Figure 62.) Now attempt 

 to " suck " the water out of the bottle through the 

 open tube. Does it come out freely? Pull out the glass plug. 

 Does it come out any better ? If so, why ? 



Anything that has weight must exert pressure upon the 

 surface upon which it rests. The air has been found to have 

 weight, and therefore it must exert pressure at the surface 

 of the earth. It is a gas; and since the particles of a gas 

 easily move over one another, this pressure must be exerted 

 equally in all directions. 



We do not feel the pressure of the atmosphere because the 

 pressure inside us balances the pressure from without. If 

 two eggshells, with their contents removed one of them 

 with the holes left in it, and the other completely sealed - 

 should be sunk to a considerable depth in water, which one 

 would be crushed by the pressure of the water, and which 

 would not ? This illustrates why objects on the surface of 

 the earth are not crushed by the pressure of the air. 



