CHAPTER X 

 GRADES AND SELF-CLEANSING VELOCITIES 



IN the early days of sewer-construction the fact that sewers 

 could be kept clean by any other method than by periodic 

 sweeping was scarcely appreciated. Sewers were a subject 

 not fit even for discussion, much less for the professional inter- 

 ests of any except the meanest laborers. Sewers were neces- 

 sary, were to be taken for granted, but were not to be made a 

 topic of public conversation. To this public attitude towards 

 sewers it is undoubtedly due that the principles of hydraulics, 

 early studied in the case of rivers by the most eminent scien- 

 tists, have been so tardily applied to sewage-flow, and have only 

 recently been recognized in determining the size, shape, grade, 

 etc., to make the sewer best suit its intended purpose. 



Baldwin Latham gives examples of defective house-drains 

 said to be still in use in London houses. Fig. 45 shows a 



defective section of a sewer car- , . __^ 



rying storm-water and sewage, 

 which has been in use in 

 Ithaca for many years. Many 

 examples could be found of 

 similar faulty construction, of 

 broad flat inverts, of rough sur- FIG 



faces, of open joints, and of 



grades not sufficient to carry along the matter in suspension. 

 It is, however, enough to point out that such imperfectly con- 

 structed sewers have been the rule in the past, and that only 

 within the latter half of the last century has the relation 

 between the hydraulic elements concerned and a clean non- 

 depositing sewer been recognized. Now it is known that 



with a sufficient velocity and depth any material that has 



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