118 FIRST YEAR SCIENCE 



to cork before removing entirely from the heat. Set the tin thus 

 corked upon the desk and observe. What happens as the steam con- 

 denses? Why? 



Experiment 64. By means of an air pump exhaust the air from 

 a pair of Magdeburg hemispheres. Now try to pull the hemispheres 



apart. It cannot be done as easily as 

 before the air was exhausted. Why ? 



Experiment 65. Fill a glass tumbler 

 P. CA even full of water and press upon it a 



piece of writing paper. 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 posi- 

 tion, insert the point of a pencil between the 

 paper and the rim of the tumbler. What happens? Fig. 55. 



Anything that has weight must exert pressure upon the 

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

 have weight, therefore it must exert pressure at the sur- 

 face of the earth. Air is a gas and its particles easily 

 move over each other, therefore this pressure is exerted 

 equally in all directions. No one feels the pressure, how- 

 ever, because the air is within us as well as about us. 

 Those that have measured this pressure find that it is 

 about fifteen pounds to the square inch at sea level. If 

 two egg shells from which the insides had been removed, 

 one of them with the holes left in it and the other com- 

 pletely sealed, were sunk to a considerable depth in water, 

 which one would be crushed and which one would not ? 

 This illustrates why we are not crushed by the pressure 

 of the air upon us. 



56. Decrease of Volume due to Pressure. Experiment 66. 



In a Mariotte's tube cause about a centimeter of mercury in tte short 

 arm to balance the same amount in the long arm. The pressure 

 inside the short tube will then be equal to that outside the long tube 



