108 ELEMENTARY GENERAL SCIENCE CHAP. 



boil a little water in the can. After the water has been boil- 

 ing for some time, so that the can is practically filled with 

 steam, remove the can from the flame, and quickly put in the 

 stopper as tightly as you can. After a few minutes the can 

 will collapse. 



The explanation of the effect produced in this experiment is 

 that as the can cools the steam inside is condensed into 

 water, and so occupies a much smaller volume. The pressure 

 which the steam exerts on the inside of the can is thus re- 

 moved, while the pressure of the air on the outside remains 

 practically the same, the result being that the can is crushed. 

 At the sea-level, under ordinary conditions, the pressure of the 

 air is 15 Ibs. on every square inch. 



The following experiments also illustrate effects of atmospheric 

 pressure : 



EXPT. 103. Moisten a leathern sucker, press it upon a 

 flat stone, and show that it can only be pulled off with 

 difficulty, owing to the pressure of the atmosphere upon its 

 upper surface. 



EXPT. 104. Dip the open end of a glass syringe or squirt 

 into a bowl of water. Pull up the piston, and see that the 

 water follows it, owing to the pressure of the atmosphere 

 upon the surface of the water in the bowl. The action of a 

 pump is very similar to this. 



EXPT. 105. Take a tumbler or cylinder with ground edges 

 and completely fill it with water. Place a piece of stout 

 writing paper across the top and invert 

 the vessel. If the air has been care- 

 fully excluded from the cylinder the 

 water does not run out (Fig. 47). Think 

 what keeps the paper in its place. 



Principle of the Mercurial Baro- 

 meter. It has been seen that the air has 

 weight, and that it exerts great pressure 



ir,. 47. Experiment to on the earth's surface ; we have now 

 of'the 're 8BUro fco leam hovv this P ressure is measured. 



EXPT. 106. Procure a barometer 



tube and fit a short piece of india-rubber tubing upon its open 

 end. Tie the free end of the tubing to a glass tube about 

 six inches long open at both ends. Rest the barometer 



