THE HOME WATER SUPPLY 51 



The weight of air presses down on the water in the cistern 

 and the pressure is exerted all through the water, so that, at 

 the opening of the pipe, water is forced up the pipe and into 

 the cylinder of the pump. When the piston of the pump is 

 raised, a vacuum is formed in the cylinder and water is 

 forced up to fill this space by the weight of the air on the 

 surface of the water in the cistern. 



As there is a limit to the weight of air, so there is a limit 

 to the height to which water may be pumped by the com- 

 mon suction pump. This limit is the height of a column of 

 water which is one inch square and weighs fifteen pounds. 

 This height is found to be about thirty-two feet. Air will 

 not force water higher than this, because the weight of this 

 height of water is just balanced by the weight of air extend- 

 ing up as high as it goes. 



DEMONSTRATION n 



Purpose. To show that air has weight. 



Apparatus. Air pump, bell jar with open top, rubber mem- 

 brane. 



Directions. Tie a piece of rubber membrane over the open 

 top of a bell jar. Place the bell jar on the stand of an air pump 

 and remove the air by means of the pump. 



LABORATORY EXERCISE 7 



Purpose. To show that air has weight and hence exerts pres- 

 sure. : 



Apparatus. Test tube, glass tubing about six inches long, 

 glass tumbler, piece of paper, rubber tubing about a foot and a 

 half long, pint milk bottle, hard-boiled egg with shell removed. 



Directions, i. Fill a test tube with water. Place your 

 thumb over the end and invert it in a dish of water. Remove 

 your thumb after the end of the tube i.s under water. Why does 

 the water stay in the tube ? 



2. Put a piece of glass tubing in water till it is submerged. 

 Place your finger over the upper end and remove the tube from 



