50 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK i. 



CHAPTER III. 



PUMPS. ATMOSPHERIC RAILWAYS AND LETTER TUBES. 



I. PUMPS. ATMOSPHERIC PRESSURE EMPLOYED IN THE ELEVATION 



OF WATER. 



A PUMP barrel, or cylinder, in which a piston causes a vacuum 

 by an up-and-down movement ; a pipe of more or less length, 

 communicating at one end with the lower part of the body of the 

 pump, and at the other with a reservoir of water or a well, in which 

 pipe the air is rarefied at the same time and by the same action as 

 the air in the body of the pump. Such are the principal parts of the 

 suction-pump as it is used in numerous instances, and principally for 

 domestic purposes. The principle on which the raising of the water 

 depends is, as indicated in the Forces of Nature, that of atmospheric 

 pressure, which exercises its whole force on the surface of the water 

 in the reservoir, whilst it is nil, or at least reduced, in the interior of 

 the pipes and in the portion of the pump situated below the piston. 

 Fig. 30 shows how a pump of this kind is generally fixed above 

 a well when the depth of the well is less than 7 or 8 metres below 

 the point where the water flows from the pump. It will be seen 

 by examination of the drawing how the up-and-down motion of the 

 piston first exhausts the air in the cylinder and then continues to 

 raise the water. In the piston is a valve, or in other words a door 

 opening upwards only ; at the bottom of the barrel is a similar valve 

 which also only allows a passage in one direction. When the piston 

 descends, the air or water in the barrel is compressed between it and 

 the bottom valve, and not being able to escape downwards it passes 



