CHAPTER VII 

 THE SUCTION-PUMP. ATMOSPHERIC PRESSURE 



You have noted that man discovered a very long time 

 ago that a good way to get pure drinking water is to dig 

 for it. But having dug for it and found it, he had a good 

 deal of trouble in getting it out of the hole. The ancient 

 way was to lower a pail, and 

 then to pull it up. There was 

 considerable danger of falling in 

 while you were doing this. So 

 they built curbs about the wells. 

 Then some bright person dis- 

 covered that you could pull the 

 pail up more easily by winding 



... . . f FIG. 22. The windlass. 



it up on a revolving bar which 



you turned with a handle. This was the invention of the 

 windlass (see Fig. 22). But it was a long time after wind- 

 lasses were invented before any one happened to get the 

 idea of a pump. 



To understand how a pump works you must understand 

 atmospheric pressure. Atmosphere is simply another word 

 for air. You may have heard that air at sea-level exerts 

 a pressure of fifteen pounds to the square inch. This at- 

 mospheric pressure is due to the fact that this mixture of 

 gases we call air extends for some miles upward from the 

 earth, and that this great sea of air, at the bottom of which 

 we live, has a certain weight. It is so evenly distributed 

 about us, and w\. are so used to it, that we are not con- 



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