226 Introduction to the Study of Science 



many city systems. When water is being used freely for sprin- 

 kling lawns or in laundry work on basement or first-floor levels, 

 often little or none is available on higher floors in that locality. 

 This is especially the case when the pressure is not increased by 

 special means, such as the force pump, or when the reservoir is 

 not high enough to give sufficient gravity pressure at all times 

 throughout the system. 



91. How pressure is secured. It appears in the study of 

 water supply that many cities, as Los Angeles, Denver, and 

 New York, depend upon the elevation of the source or the 

 distribution reservoir for the necessary pressure for all ordinary 

 demands. In the case of New York City the Cats kill supply 

 is sufficiently elevated to force the water to a height of 240 feet 

 above sea level, equivalent to about twenty stories. In build- 

 ings exceeding this height, special means are provided by the 

 owners to secure adequate pressure for usual service and for 

 emergencies. The elevation of the Los Angeles supply, which 

 is more than 4000 feet, gives a pressure which, after deducting 

 for loss due to friction in conduits, is easily sufficient for all 

 possible needs. 



Many cities obtain their water from a source which makes 

 the use of natural or gravity pressure impossible. Where a 

 suitable hill is available it may be made the site of a storage and 

 distributing reservoir, to which the water is pumped by force 

 pumps. In many places where the supply is provided by deep 

 wells, by a lake or a running stream, it is delivered directly into 

 the mains and distributed at the desired pressure by pumping 

 plants. In the country pressure may be secured by a supply 

 tank elevated on a trestle out of doors or located in the attic of 

 the house. Water is delivered to the tank by a force pump 

 operated by a windmill or a gasoline engine. Sometimes water 

 may be supplied to the tank by a hydraulic ram. If the tank is 

 placed in the basement, it may be made air-tight and water 

 delivered at desired pressure by means of a compressed air 

 machine or pump. 



