The Supply and Uses of Water 229 



The standpipe. During the daytime, water is usually drawn 

 in largest quantities at all points in a system. The flow conse- 

 quently becomes very feeble at the places farthest from the 

 source of supply. To remedy this a water-tower or steel stand- 

 pipe is often used in long pipe lines. This is a sort of supple- 

 mentary reservoir, which stores up excess water when the 

 demands are relatively small, and furnishes additional supply 

 when the demands are greatest. In its operation it resembles 

 the air reservoir of a force pump. The compressed air cushion 

 in the standpipe (cf. page 231) assures a steady and sufficient 

 flow at all points. 



94. How water transmits pressure. The way in which 

 water in a closed container, such as a .tank or a system of pipes, 

 transmits pressure was interestingly demonstrated by Pascal 

 (1623-1662), a French scientist. He filled a small barrel with 

 water and fitted in the bunghole a vertical tube many feet 

 in length. Into this tube he poured water until the pressure 

 became so great as to burst the barrel. The weight of the water 

 in the tube was comparatively small, yet it exerted enough 

 pressure distributed with equal intensity in all directions or 

 over the entire inner surface of the barrel, to cause the barrel 

 to burst. The important point is that when pressure is applied 

 to any unit area of a liquid in a closed container, it is transmitted 

 in exactly the same amount against every unit area of the container 

 in contact with the liquid. The pressure is exerted also in a 

 direction which is perpendicular to the area in contact. 



Suppose that the area of the cross section of the tube is one 

 square inch, then the pressure of a column of water twelve inches 

 high would be 0.434 of a pound. This pressure is exerted upon 

 each square inch of the inner surface of the barrel and in a 

 direction which is perpendicular to the surface at every point. 

 If the height of the column is increased to two feet, the pressure 

 per square inch is 2 X .434, or .868 pound. In short, increasing 

 the height of the column increases at exactly the same rate the 

 pressure upon each unit area in contact with the liquid. 



