GENERAL CONSIDERATIONS 11 



tile are laid, to carry them separately to one or more out- 

 falls. 



The question of drainage of cellars and low yards through 

 the storm- water sewers is a serious one best settled by expediency. 

 Like exceptionally low basement fixtures, they require for their 

 individual accommodation a general lowering of the sewer-line 

 or some special pipe or arrangement. The question resolves 

 itself into one of the general versus the individual good : whether 

 it is just to add to the general cost of the whole work for the 

 peculiar benefit of one or two. When the whole section is low 

 and relief can be given only by special means, then, as citizens, 

 the householders are entitled to it, but it is probable that the 

 individual case is more fairly neglected. 



The combined system is not adapted to any case where 

 the sewage has to be pumped, treated by chemicals, or dis- 

 posed of on land; the rain-water must be kept out of the sewers, 

 and no ground-water or other unpolluted water allowed to 

 enter in all such cases. 



PROBLEMS 



1. Given the formula of hydraulics Q=AC^RS, define carefully each 

 of the factors used, and notice particularly the units employed. 



2. Given the formula of hydraulics V = C^RS, show algebraically 

 that on the same grade a 6-inch pipe flowing half full has a less velocity 

 than a 1 2-inch pipe half full. Express numerically if C = ioo and S = i 

 foot in 100 feet. 



3. Compute the velocities in a pipe if, in one case the pipe flows full, 

 in another, half full, and in another one-quarter full. Assume a 1 2-inch 

 pipe on a grade of 26 feet to the mile, and that C = 100. 



4. Assume an area of one square mile on which there are 12,000 persons. 

 If the rainfall is assumed to be at the rate of 2 inches per hour, how much 

 water falls on the area in cubic feet per second? If the domestic sewage 

 flow is at the rate of 100 gallons per head per day, what is the sewage 

 flow? If one-third the rainfall reaches the sewer, what is the ratio of 

 storm-water flow to house-sewage flow? 



5. If a 1 2-inch pipe-sewer laid, costs 90 c. per running foot and brick- 

 work for a 4-foot circular sewer costs $20 per thousand, and excavation 

 in trench costs 60 c. per cubic yard, compare the cost of the 1 2-inch domestic 



