232 GENERAL SCIENCE 



also vary. As the height of water in the reservoir changes, 

 the pressure in the houses will also change. 



Water pressure is also reduced by the friction of the 

 flow of the water through the pipes, joints, and valves. 

 Again, if many people are using water at the same time, 

 the pressure will be lessened because the pipes are not 

 usually large enough to carry a full pressure supply for 

 all at the same time. So the three conditions upon which 

 water pressure is dependent are the height of water in 

 the reservoir, friction in the pipes, and the number of 

 people using water at one time. If several people on 

 the first floor of a building are drawing water, those 

 on the upper floors may not get any, as trie pipes in the 

 basement are not usually large enough to carry sufficient 

 water to make it flow from all the faucets at the same 

 time. 



158. Equal Pressure. Large cities try to meet the 

 difficulties discussed in 157 by having reservoirs on 

 various hilltops; or if hills are not sufficiently numerous, 

 standpipes are erected at various distances from the 

 reservoir, or pumping station. If there are a number of 

 reservoirs, the friction caused by water flowing through 

 long pipes is avoided and a more constant pressure and 

 supply are secured. If the reservoir and standpipe 

 system is used, the water flows from the reservoir into 

 the standpipe when not many people are drawing water. 

 During the hours when a large quantity of water is 

 needed, it flows from the standpipe because the distance 

 of flow is less and hence the friction is not so great as when 

 the water flows from the distant reservoir. Standpipes 

 are also often combined with the pumping system in order 

 to insure normal pressure in the parts of the city which 

 are distant from the pumping station. 



