SEWER PLANS 



203 



of laterals and lines of equal size it is best to combine them 

 into a main as soon as possible, rather than to have a number 

 of lines of about the same capacity. In algebraic terms, it is 

 cheaper to build a single line of nx capacity than to build n lines 

 of x capacity. 



Figs. 54 and 55 illustrate the point, the length of the sewers 

 being the same in both cases; but as the length of the small 

 laterals is greater and that of the mains less in Fig. 55 than in 

 Fig. 54, the former is the more economical arrangement. Further, 

 the laterals will have a better grade, that is, the grade will be 



V 



FIG. 55. 



placed where it is most needed, this resulting from the fact 

 that, since the main in Fig. 54 is larger than that in Fig. 55, 

 it will not require as great a grade for the same velocity. Com- 

 pare also in Fig. 56 the two sides of the diagram, illustrating 

 two ways of laying out the pipes. 



On the other hand, in order to maintain in the sewers as 

 uniform a velocity as possible, and in order to avoid deposits, 

 wherever the sewage from a hillside discharges into a flat it is 

 better to carry the sewage along contours than perpendicular 

 to them. This will not increase the length nor, as a rule, the 

 sizes of the sewers, since every street must have a sewer, and 

 since where this arrangement is desirable the grades are ample 



