PRESIDENTIAL ADDRESS. 



51 



The sewer, which is one of the largest in the world, is 

 six and a quarter miles long, its greatest width being twelve 

 feet three inches, and greatest height nine feet; it drains 

 an area of 38,000 acres, which includes some of the closest 

 settled suburban districts, with an estimated population of 

 300.000; the gradient of the sewer is very flat, being only 

 1 in 3650 from the manhole at the syphon under Cook's 

 River to the outfall. 



The wastes from the woolscouring works and tanneries 

 at Botany, which were formerly discharged into Cook's 

 River, are now turned into the sewer, thus removing a 

 source of pollution from the river which was causing trouble. 

 The sewer was completed in 1916 at a cost of £516,158. 



Many engineering difficulties were met with in its con- 

 struction, especially where it passed through wet sand, 

 which was in the deepest part of the cutting, at a point 

 where the invert of the sewer is 68 feet below the surface. 



A number of constructional difficulties were encountered 

 in forming the outlet channels to the ocean, special con- 

 trivances being required to prevent the waves rushing up 

 the sewer. A large air chamber with a shaft to the surface 

 was constructed a hundred feet back from the edge of the 

 ocean cliff. The bottom of the sewer at this air chamber 

 is two and a quarter feet below spring high-water level. 

 The sewer discharges into a basin constructed with wave 

 traps, from which two channels have been driven, diverging 

 like the sides of the letter "V," and dipping so that they 

 run out into the ocean from the face of a submerged cliff 

 300 feet from the shaft, at such depth as to give twenty- 

 three feet of water above the top of each outlet at spring 

 tide. The rock through which these channels were driven 

 proved sound and free from fissures, so that tunnelling was 

 carried on without difficulty within five feet of the ocean. 

 The tunnels having been lined with concrete, a number of 



