CITY WATER SUPPLY 233 



159. The Cost of City Water. Families may have 

 to pay from one dollar to ten dollars per year for water. 

 This is not very much per gallon, but when the total cost 

 of a large city is ascertained it amounts to millions of 

 dollars. Where the gravity system is used the cost is 

 very small, as not much work is needed after the pipes 

 are once laid. But where the cities are in a level country, 

 or where the source of water is no higher than the city, 

 the cost of water is greater because of the necessity of 

 pumping it and often of purifying it. 



To get some idea of the work done in pumping water 

 for a city, let us consider the Pennsylvania Water Co., 

 which supplies water to about 30,000 people, principally 

 in Wilkinsburg, Pa. A gallon of water weighs approxi- 

 mately 8.3 pounds. The work done by a pump in raising 

 a gallon of water 560 feet to the filtering plant is 8.3 X 

 560, or 4,648 foot pounds. The company pumps 8,000,- 

 ooo gallons every 24 hours, and the work done in raising 

 it to the filtering plant is 8.3 X 560 X 8,000,000, or 

 37,184,000,000 foot pounds. The work that can be 

 done by one horse power in 24 hours is 550 X 60 X 60 X 

 24, or 47,520,000 foot pounds. So the number of horse 

 power required to pump that water is approximately 

 37,184,000,000 -4- 47,520,000, or 782 H. P. An engine 

 of about 1,000 H. P. would be required to pump the 

 8,000,000 gallons per day up to the filtering plant. After 

 the water is filtered, it is destributed by the force of 

 gravity to the various houses in the valley below. 



160. Water Supply and Forests. There are two 

 causes which influence the maximum flood stage of most 

 streams. 



First. The highest floods usually come when a heavy 

 snow is melted by a warm rain during the winter or early 



