132 A YEAR IN SCIENCE 



soon as a part of the air is pumped out of the glass, the 

 pressure of the air beneath the rubber is less than the 

 pressure of the air above it, and the rubber is forced 

 doAvn into the glass. 



If a piece of glass tubing is placed in a beaker of 

 water, the water will not rise in the tube. If the air is 

 then drawn out of the tube by the mouth, the water will 

 rise in the tube. When the air is withdrawn from the 

 tube by the mouth, the pressure within the tube is 

 reduced. The liquid is then forced up the tube be- 

 cause of the pressure of the air on the surface of the 

 water in the beaker. This is what happens when we 

 take soda water or lemonade through a straw. 



A test tube filled with water is closed by the thumb 

 and then inverted mouth downward into a jar of water. 

 After the thumb is removed the water does not run 

 out of the test tube, because the pressure of the air 

 on the surface of the water is sufficient to prevent this. 



If there is a limit to the amount of pressure which 

 the air exerts, there must be a limit to the column of 

 water which it will hold up. 



If, instead of a short test tube, a tube 35 feet or more 

 in length had been used, would the result have been the 

 same as that obtained with the test tube? This has 

 been tried. Careful experiments have shown that the 

 pressure of the air at sea level is sufficient to hold up 

 about 34 feet of water. 



If the area of the opening at the base of the tube 

 is one square inch and the height of the column of 



