THE COMMON PUMP OR LIFTING PUMP 



189 



live in cities where running water is always present in the 

 home cannot realize the hardship of the days when this 

 " ready-made " supply did not exist, but when man labori- 

 ously carried to his dwelling, from distant spring and stream, 

 the water necessary for the daily need. 



What are the characteristics of the air which have enabled 

 man to accomplish these feats ? They are well known to us 

 and may be briefly stated as follows : 



(1) Air has weight, and I cubic foot of air, at atmospheric 

 pressure, weighs \\ ounces. 



(2) The air around us presses with a force of about 15 

 pounds upon every square inch of surface 



that it touches. 



(3) Air is elastic; it can be compressed, 

 as in the balloon or bicycle tire, but it ex- 

 pands immediately when pressure is reduced. 

 As it expands and occupies more space, its 

 pressure falls and it exerts less force against 

 the matter with which it comes in contact. 

 If, for example, I cubic foot of air is allowed 

 to expand and occupy 2 cubic feet of space, 

 the pressure which it exerts is reduced one 

 half. When air is compressed, its pressure 

 increases, and it exerts a greater force against 

 the matter with which it comes in contact. 



If 2 cubic feet of air are compressed to I FIG. 132. Theatmos- 



..... '. phere pressing 



cubic foot, the pressure of the compressed air downward on a 

 is doubled. (See Section 89.) ^ shes water * fter 



v the rising piston b. 



183. The Common Pump or Lifting Pump. 



Place a tube containing a close-fitting piston in a vessel of water, 

 as shown in Figure 132. Then raise the piston with the hand 

 and notice that the water rises in the piston tube. The rise of 

 water in the piston tube is similar to the raising of lemonade 



