USE OF DIAGRAMS 193 



in the gutter it will take about three-fourths of a minute more 

 to reach Stewart Avenue. In the 1 5-inch pipe on the 10 per 

 cent grade the velocity is between 12 and 15 feet per second for 

 300 feet, adding a quarter minute, or 3 minutes in all. For the 

 remaining distance the time will be 6 feet per second, requiring 

 nearly 5 minutes more, or 8 in all. This agrees with the time 

 assumed, and therefore a second rate of rainfall based on this 

 time just found will not be necessary. 



For a possible maximum with a shorter storm, if the 

 Buffalo Street lateral is considered, it will take only about 

 6 minutes for its water to reach the outfall, including the time 

 necessary for the water to reach the gutter, and the corre- 

 sponding rainfall is 3.4 instead of 3.2 as used before. For the 

 limited area drained by this lateral it is plain that there can 

 be no maximum flow brought down by this pipe. 



To illustrate the method of determining the sizes of pipes 

 for domestic sewage, assume that it is required to find the size 

 for the Northern Main in the city of Ithaca, that is, the pipe 

 coming directly to the pumping-station and taking the sewage 

 from the region north and east of Cascadilla Creek. According 

 to the map this is an area of 138 acres, and is populated at the 

 rate of about 30 per acre. The area taken is all that can ever 

 drain into the system, and represents all the future population 

 in the district. The population to be considered is 4140. After 

 studying the water-supply it is assumed that a future provision 

 of 70 gallons per head per day should be provided, that is, an 

 average daily flow of 289,800 gallons. If half of this is sup- 

 posed to flow off in 8 hours, the rate of flow in those hours 

 will be 18,110 gallons per hour, or 302 gallons per minute, equal 

 to 40 cubic feet per minute. A knowledge of the territory 

 will justify the assumption that fully 2 miles of the pipe-line 

 will be covered with ground- water, which, at the rate of 

 20,000 gallons per mile per day, will add 4 cubic feet per minute 

 to the flow, making 44 in all. By the topographical conditions 

 the grade will be the minimum, and will be determined by the 

 requirements of velocity. Looking on Plate 3, we find that 



