THE COMMON PUMP OR LIFTING PUMP 321 



realize the hardship of the days when this " ready-made " 

 supply did not exist, but when man laboriously carried to his 

 dwelling, from distant spring and stream, the water necessary 

 for the daily need (Fig. 192). 



What are the characteristics of the air which have enabled 

 man to accomplish these feats ? They may be briefly stated 

 as follows : 



(1) Air has weight. One cubic foot of 

 air, at atmospheric pressure, weighs ii ounces. 

 (See Chapter XXXVIII.) 



(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, one cubic foot of air is allowed 

 to expand and occupy two cubic feet of space, 

 its pressure 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 two cubic feet of air are compressed 

 to one cubic foot, the pressure of the compressed air is doubled. 

 The common pump or lifting pump. Place a tube contain- 

 ing a close-fitting piston in a vessel of water, as shown in Fig- 

 ure 193. Then raise the piston with the hand and notice that 

 the water rises in the piston tube. The atmosphere presses 

 with a force of 15 pounds upon every square inch of water in 

 the large vessel and forces some of it into the space left vacant 

 by the retreating piston. The common pump wovks in a similar 



FIG. 193. The at- 

 mosphere pressing 

 downward on a 

 pushes water after 

 the rising piston b. 



