98 SOME EEMARKS ON SEWERAGE SYSTEMS. 



with reference to their size, form, and inclination, in 

 order to render them self-cleansing. Their area was largely- 

 reduced, and they were so proportioned that with a 

 minimum flow there should be a minimum wetted perimeter. 

 Tor this purpose the egg-shaped sewer was designed by 

 Mr. Phillips for sewers carrying, or liable to carry, large 

 volumes of sewage. 



In smaller sewers circular glazed stone-ware or fireclay 

 pipes are generally used, the circular form being stronger to 

 resist a crushing pressure, and the wetted perimeter being 

 only slightly increased above an egg-shaped sewer of the 

 same dimensions. These sewers are now laid down to 9'' 

 and even 6'' and 4" ; but the latter dimension is not recom- 

 mended, the pipe being liable to get blocked. 



Numerous empirical formulae have been deduced for 



calculating the velocity and discharge of sewers ; but that, 



I believe, most generally used is Eytelwein's adopted from 



Chezy's, which runs, — 



7=55^2117 



where V= velocity in feet per minute 



^=fall in feet per mile 



, -, ,. 1 ,1 area of cross section. 



r = hydraulic mean depth = .; — ,; — = , — . 



•^ length 01 wetted perimeter 



It will thus be seen that, the fall being constant, V varies 

 directly as the square root of the sectional area, and in- 

 versely as the square root of the wetted perimeter. It is 

 now necessary to find a velocity that will render the sewer 

 self-cleansing. As a rule, a velocity of 2| feet per second, 

 or 150 feet per minute, is generally calculated for in 

 designing sewers. It should be borne in mind in connec- 

 ^tion with this, that the velocity of sewage will be some- 

 what dependent on its purity ; for pure water will flow 

 with less frictional resistance than thick muddy water. 



