374 Transactions. — Miscellaneous. 



In flat districts where sufficient fall has not been provided 

 these results are always to be found, and can only be obviated 

 by deep and expensive sewers leading to a pumping-station, or 

 by the adoption of the principle I now advocate. 



By pneumatic pressure an artificial head is provided, which 

 forces the contents of the intercepting sewer or pipe at a proper 

 velocity along its whole length, regardless of its inclination, 

 to the outfall. This artificial head takes the place of the 

 natural head upon a gravitation sewer, and gives the requisite 

 velocity to its flow. For example, with an ISin. pipe one 

 mile in length, discharging 318 cubic feet per minute, the 

 velocity of its contents will be 3ffc. per second. The fall in a 

 gravitating sewer to bring about this result would be 1 in 

 350, the height or head necessary to overcome the friction in 

 the pipe being 15-lft. By Shone's system we force the con- 

 tents into and along the pipe (which can be laid on the level) 

 at a pressure of 6'5o31b. per square inch, that being the 

 equivalent of a head of 15-lft., the result in both cases being 

 the same. 



In the gravitating pipe we must have a natural fall of 

 15-lft., and lay it below the hydrauhc grade-line at a probably 

 heavy expense. By Shone's system we may have no fall 

 whatever, laying the pipes upon the undulating surface of the 

 ground at the least possible cost. We must, however, supply 

 318 cubic feet of air at a minimum pressure of 6-553Ib. per 

 square inch, with which the sewage is forced into and along 

 the pipe. 



If it can be shown that to supply that quantity of air under 

 pressm-e is less expensive than to gravitate the sewage to a 

 pumping-station, and then lift it to a height corresponding to 

 the loss of head, the pneumatic system must be preferable, 

 even when other results are equal. 



We will take the schemes as proposed for draining the low- 

 lying parts of Wellington as an example. 



i3y Clark's scheme the sewage from this area was to be 

 collected by an expensive brick intercepting sewer terminating 

 at a pumping-station where the sewer would be 9ft. below 

 high-water mark. Thence it was to be pumped through a 

 cast-iron main nearly a mile in length to a height of 37ft. 

 above the intercepting sewer, where it would join the sewage 

 flowing by the gi-avitation sewer which was to drain the higher 

 portions of the city, and would thence flow through a tunnel 

 a mile and a quarter long to the sand-hills near Lyall's 

 Bay. 



By the pneumatic system cast-iron mains are substituted 

 for brick intercepting sewers, and are only laid sufficiently 

 below the surface of the ground to keep them out of sight 

 and free from damage. Into tliis cast-iron main the sewage 



