1890-91.] 



A CONSIDERATION OF SEWKRAGE SCHEMES. 



151 



of electricity, Styled electrolysis, which is also fully described, but time 

 precludes me from givin<^ a description of them in the present paper. 



III. Under the third head I wish to point out some general defects in 

 the sewerage and sewage disposal works of towns and cities. The first 

 defect relates to the size and fall required by sewers. These should be 

 considered with a view to the amount of sewage required to be carried 

 by them. They should be regulated to such a size that they will run 

 from ^ to ^ or ^ full. Let me illustrate what takes place when a 

 sewer is too large. We will suppose a sewer is constructed 4 feet in 

 diameter, and that the depth of sewage flowing in it is one foot, — much 

 less than this is very common. I was informed that on Perth street a 

 sewer 4 feet in diameter was being constructed, and I am satisfied that 

 the depth of sewage flowing in it will not average one foot. 



Firs.l 



F.<).m 



Tic^M. 



Owing to the small volume of sewage coming in contact with the com- 

 paratively large surface of sewer pipe as in Fig. I., the friction will be 

 comparatively greater, and though the fall might be suf^cient to produce 

 a scouring action were the sewer running full or half full, the motion 

 will be retarded till deposits begin to take place as in Fig. II. This, you 

 will observe, increases the frictional surface and still more retards the 

 flow ; when sedimentation will go on more rapidly as in Fig III., IV. 

 and V. Here you might suppose that the evil would begin to cure 

 itself, as the restriction of the passage, as in Fig. VI., would cause an 

 increased velocity, but not so ; increased velocity cannot be obtained 



