52 . T. H. HOUGHTON. 



holes twelve inches apart were drilled in the rock round 

 the circumference of each tunnel face, to within eighteen 

 inches of the water. Two large centrally situated holes 

 were also drilled, and all the holes charged with gelignite. 

 Detonators with copper wires were inserted, and connected 

 with cables running to a battery above high-water level. 

 The water was then slowly admitted from a hole driven 

 through the face into the sea, until it rose in the tunnel to 

 the level of the ocean, and when the water pressure on 

 both faces of the rock partition became equal, the charges 

 were fired. Weather conditions being favourable, a diver 

 descended from the end of the wall under which the tunnels 

 were driven. In the case of one tunnel he found the outlet 

 clear, the rock having been cleanly cut out, and blown 

 outwards. The other tunnel, while the explosive had done 

 its work effectively, was blocked with large fragments of 

 rock, which remained lodged in the tunnel mouth. These 

 masses of rock had to be drilled under water and broken up* 

 with explosives before the tunnel mouth could be cleared. 



The sewage on the northern side of the harbour is dealt 

 with at Ohatswood, Balmoral, and Folly Point by septic 

 tanks and filters, the effluent being discharged into branches 

 of Middle Harbour. The Folly Point works were originally 

 constructed to treat sewage by lime precipitation and sub- 

 sequent filtration; soon after completion the method was 

 altered, the precipitation tanks being converted into septic 

 or liquifying tanks, and a portion of what were originally 

 sand filters converted into percolating filters. 



Recently exhaustive experiments have been made with 

 a new method of sewage purification termed Sludge Activa- 

 tion, and I understand that the system is about to be used 

 at Folly Point instead of the septic or liquifying tanks, as 

 the experiments have demonstrated that it is more effective, 

 and possibly causes less annoyance to those living in the 

 immediate neighbourhood of the works. 



