The Drainage, of Melbourne. G7 



area of 200 acres for filtration, in addition to 1800 acres for 

 irrigation; whereas the essay provides 120 acres only for the 

 one process alone. 



18. But there is yet another question which concerns the 

 proper filtration of the sewage in the locality selected, even 

 if a sufficient area should be taken up — How far is the level 

 of the land suitable 1 Drains are to be laid six feet below 

 the surface, and these drains must be sufficiently above the 

 water level of the bay to admit of the effluent water flowing 

 off at all times, besides allowing a slope in the drains them- 

 selves to give the requisite discharge. Is the ground high 

 enough for this ? If it is not, it could certainly be raised to 

 the required level ; but does the estimate provide for this ? 

 A sum of £2500 is shown as the cost of preparing filtering 

 ground, equal to nearly £21 an acre for the 120 acres at first 

 required. According to a report by Mr. Rawlinson, "the 

 works at Merthyr cost £230 per acre, while those at 

 Kendall cost £280 per acre."* On what data then is £21 an 

 acre considered sufficient in the case of Melbourne, not only 

 to underdrain the land and prepare the surface, but also to 

 raise it ? 



19. With regard to the sewers, a few remarks may be now 

 usefully made. It is stated in the essay — " The smaller street 

 drains, being designed to carry off the sewage immediately, 

 before it has time to become putrid, do not require flushing 

 in general, but they should be washed out occasionally by 

 turning on the Yan Yean water into them to assist the 

 ventilation."*|" Flushing would thus seem to be considered 

 necessary to remove putrid matter only, and not solid sub- 

 stances generally, putrid or otherwise, which are liable to 

 lodge in drains. It is usually found that, as the flow of 

 sewage in the smaller drains and sewers is intermittent, they 

 are particularly liable to be stopped up by paper, hair, grease, 

 &c, and that frequent flushing is necessaiy.j Water from 

 baths is especially suitable for flushing house drains and the 

 smaller sewers to remove these obstructions, since it flows 

 off in a rush ; yet the essay says it is not to be used. It 

 is also most plentiful in summer, when Yan Yean water 

 from the mains can least be spared, and flushing is most 

 required. 



* " Proceedings of the Institution of Civil Engineers," vol. 48, p. 246. 



t See paragraph 31 of the essay. 



\ Baldwin Latham's Sanitary Engineering (1878), pp. 184, 286, and 482. 



