XIV. J. DAVIS. 



40 ft., and its outlet 19J ft. above the datum, which latter is 

 1*125 feet above mean high water. The sewer is oval in section, 

 built in- concrete, with brick arching round the upper half, 

 and ranges in size from 4 ft. 6 in. by 3 ft. 6 in. to 3 ft. 3 in. 

 by 2 ft. 2 in. The grades vary from 1 in 540 to 1 in 750. The 

 level at the outlet is kept high enough above high water to allow 

 of the sewage passing through its various stages of treatment by 

 gravitation before being discharged into the harbour. 



Passing under the high land along Alfred Street, the tunnel in 

 places is 260 ft. deep. The sub-mains, however, are at a lesser 

 depth, their contents reaching the main server through drop-pipes 

 built in the sides of segmental shafts. There are twelve of 

 these deep shafts on the main line, substantially built of brick 

 and concrete, which are provided with wrought iron ladders 

 and all appliances necessary to facilitate inspection, flushing, 

 and ventilation. 



The sub-mains are either brick and concrete oval sewers or 

 stoneware pipes, varied with the requirements of the area 

 drained. The chief sub-mains in North Sydney are those from 

 Blue's Point and Lavender Bay, and, like the main sewers, these 

 are oval sewers in tunnel through rock. 



To intercept the drainage of the southern half of Mosman, a 

 branch sewer, 185 \ chains long, is now in course of construction. 

 It will rise near Little Sirius Cove, and, after traversing the 

 harbour slopes as an oviform sewer in tunnel, finally discharges 

 into the main sewer in Alfred Street. 



On this sewer a length of 85 chains will be built of pipes 

 constructed on the "Monier" system. The object of this 

 system of construction is to increase the strength of structures 

 by the judicious insertion of iron rods in cement ; the effect is to 

 admit of lightness and economical proportion in arches, floors, 

 or other works, as the case may be. On the Mosman sewer, 

 oval pipes 3 ft. 1 in. in length and of 2 ft. 5 in. by 1 ft. 9 in. cross 

 sectional dimensions are to be built on this principle; a lattice of 

 wrought iron is inserted in the cement of which the pipes are 



