6Q The Drainage of Melbourne. 



ence with irrigation in India would show that no culturable 

 soil could absorb anything like so much , pure water even, 

 day after day. Sand might perhaps absorb this quantity of 

 water, but not so much sewage for a continuance, I should 

 say. 



16. Again, the depth of soil through which the sewage is 

 to pass being 6 feet, giving 9680 cubic yards of soil per acre, 

 from 2 2 J to 45 gallons of sewage, or an average throughout 

 the year of SO gallons, would have to be purified daily by 

 each cubic yard of soil. On this point, Mr. Denton states 

 that soil has a cleansing power varying from 4 to 12 '4 

 gallons of sewage per cubic yard per diem. Dr. Frankland, 

 who made some experiments for the Rivers Pollution Com- 

 missioners, .stated before the Institution of Civil Engineers 

 that the average result obtained by him was 9*6 gallons per 

 cubic yard in 24 hours. In one case a good soil purified 

 so much as 15 2 gallons for a time, but there were indica- 

 tions that it would not keep up this high rate ; while another 

 soil failed entirely to purify sewage ; sand purified 5*6 

 gallons per cubic yard.* These results (which, it should be 

 noted, were obtained in the laboratory, and are, therefore, 

 more favourable than can be expected in practice) are very 

 different from the 30 gallons per cubic yard to be purified 

 at the filtering area provided for Melbourne, according to 

 the particulars given in the essay. 



17. Comparing, then, either the area provided in the essay 

 with the population, or the quantity of soil with the volume 

 of sewage to be purified by it, and taking as an example the 

 case of Merthyr Tydvil (which is referred to in support of the 

 recommendations made in the essay), the filtering area pro- 

 vided for Melbourne appears wholly inadequate. Mr. Denton 

 recommends, where suitable land can be obtained at a fair 

 price, the sewage should be disposed of ordinarily by irriga- 

 tion, and that an area for intermittent downward filtration 

 should be added as a kind of safety valve. In this way 

 only, he considers, the loss which is inseparable from any 

 other mode of disposing of sewage, " may be turned into a 

 profit.""f He says one acre for 1000 persons should be set 

 apart for filtration, out of one acre for 100 allowed for 

 irrigation.^: This would give, for a population of 200,000, an 



* "Proceedings of the Institution of Civil Engineers," vol. 48, pp. 192, 193. 

 f Bailey Denton's Sanitary Engineering, p. 339. 

 % Ibid, p. 340. 



