Weather Study 



877 



Composite crystal; high cloud at 



center and medium high cloud 



at the border. 



Photomicrograph by W. A. Bentley. 



LESSON CCXXI1 

 EXPERIMENTS TO SHOW AIR PRESSURE 



Leading thought The air presses equally 

 in all directions. 



Experiment I To show that air presses 

 upward Fill a tumbler which has an 

 unbroken edge as full of water as possible. 

 Take a piece of writing paper and cover the 

 tumbler, pressing the paper down firmly 

 upon the edge of the glass. Turn the glass 

 bottom side up and ask why the water does 

 not flow out. Allow a little air to enter; 

 what happens? Why? Turn the glass 

 filled with water and covered with paper 

 sidewise ; does the water flow out ? If not, 

 why? 



Experiment 2 To show that air passes 

 downward Ask some of the boys of the 

 class to make what they call a sucker. 

 This is a piece of leather a few inches 

 across. Through its center a string is 

 drawn which fits very closely into the leather and is held in place by a very 

 flat knot on the lower side. Dampen the leather and press it against any 

 flat surface, and try to pull it off. If possible, place the sucker on a flat 

 stone and see how heavy a stone can be lifted by the sucker. Ask why a 

 sucker clings so to the flat surface. If a little air is allowed to get between 

 the sucker and the stone, what happens ? Why ? 



Hints to the teacher regarding the Experiments The water is kept in the 

 tumbler in Experiment i by the pressure of the atmosphere against the 

 paper. If the tumbler is tipped to one side the water still remains in the 

 glass, which shows that the air is pressing against the paper from the side 

 with sufficient force to restrain the water, and if the tumbler is tipped 

 bottom side up it shows the air is pressing upward with sufficient force to 

 keep the water within the glass. 



In the case of Experiment 2, we know that the leather pressing upon the 

 floor or on the stone is not in itself adhesive, but it is made wet simply so 

 that it shall press against the smooth surface more closely. The reason 

 why we cannot pull it off is that the air is pressing down upon it with the 

 force of about fifteen pounds to the square inch. If the experiment is per- 

 formed at sea level, we should be able to lift by the string of the sucker a 

 stone weighing fifteen pounds. The reason why the water falls out of the 

 tumbler after a little air is let beneath the paper, is that then the air is 

 pressing on both sides of the paper ; and the reason why the sucker will not 

 hold if there is any air between it and the stone, is because the air is pressing 

 in both directions upon it. 



Supplementary reading The Wonderbook of the Atmosphere, Houston, 

 Chapters III, IV, V. 



