ATMOSPHERIC PRESSURE 



35 



FIG. 19 



Fig. 18 shows this same principle applied, though in this 

 case the vacuum is formed in the tube by raising the piston p. 

 Explain this. 



Experiment 33. Fill a small-mouthed bottle with water. 

 When full, pour out the water and note that it comes in spurts. 

 Explain. Now do the same with a bak- 

 ing-powder can. Does the water run 

 out in spurts? Explain the difference. 

 In pouring any liquid from a ves- 

 sel through a small opening, a small 

 amount will run out ; this causes a 

 partial vacuum within the vessel, into 

 which the air forces itself and checks 

 the flow of liquid while passing through 

 the opening. The spurting flow thus 

 caused is more marked in thicker liq- 

 uids, like molasses and oil. To get 

 a steady flow, an opening or vent is 

 sometimes made in the vessel above the liquid surface ; through, 

 this the air may run constantly, as the liquid runs out in an even 

 stream below. 



Experiment 34. Fill a tumbler with water and cover it with a 

 piece of stiff paper. Holding the paper in place, quickly invert 

 the tumbler and hold it as in Fig. 19. Why does not the water 

 A B run out ? Pull down one corner of 



the paper, still holding the tum- 

 bler inverted. What difference do 

 you note? How do you account for 

 this? 



Experiment 35. Into a U-shaped 

 tube pour mercury, as in B (Fig. 20). 

 Now tip the tube till the mercury 



comes to one end, and cover that end with the finger. Keeping 

 the finger tightly over the end, return the tube to the position 

 in A. Explain what you observe. 



Many other simple experiments may be performed, especially 

 if an air pump is used. 



FIG. 20 



