22 On the Principles and Methods [Jan., 



sewage applied by way of irrigation to the surface of undrained 

 clay land, or to water-logged free soil lacking natural drainage, the 

 earth will become sodden, and liable to create a malaria ; but with 

 a perfect system of under-drainage (designed with relation to surface 

 irrigation) at the first description of soil, and natural drainage in 

 the second, sewage-irrigated land may be rendered perfectly harm- 

 less." From time to time complaints have been raised that land 

 irrigated with sewage -water was offensive to the surrounding 

 neighbourhood ; the observations of the British Association Com- 

 mission, however, tend to prove that in most cases the application 

 of the sewage for irrigation has not teen attended with any apparent 

 change in the sanitary condition of the district, whilst in several 

 instances there has been a marked improvement. Generally speak- 

 ing, too, no objection appears to have been made to the application 

 of sewage in this manner, and where such objections have been 

 made, on the ground that the application was offensive and in- 

 jurious, they do not appear to have been supported by medical 

 authorities, and in several instances they have ceased. 



The quantity of sewage that may be applied with advantage to 

 an irrigated area in the course of the year has been closely investi- 

 gated by Mr. Latham, and as the results of his calculations agree 

 entirely with the experience obtained at Croydon, and with the 

 experiments made by the Sewage Commission, we give them here 

 as being probably the closest approximation to exactness yet ob- 

 tained. Where it is considered desirable to apply as much sewage 

 as will be sufficient for the growth of a grass crop, without drawing 

 on the resources of the soil, 3645 tons of sewage per acre per 

 annum will be required to grow 30 tons of grass, 4860 tons of 

 sewage for a crop of 40 tons of grass, and 6075 tons for one of 50 

 tons of grass. If, however, the soil will provide half the potash 

 required, then, to grow 30 tons of grass per acre, there will be 

 required 1837 tons of sewage ; to grow 40 tons of grass per acre, 

 2450 tons of sewage ; and to grow 50 tons of grass per acre, 3062 

 tons of sewage. As 40 tons of grass per acre may be considered as 

 easy of production on a properly-regulated irrigated area, and as it 

 would not be desirable to exhaust the soil of any of its constituents, 

 Mr. Latham considers that 4860 tons per acre may be said to be 

 the right amount of sewage, and this closely assimilates with the 

 conclusions arrived at by the Sewage Commission, who reported 

 that 5000 tons of sewage per acre per annum was the right amount 

 to apply in order to get the greatest results. 



The actual value of town sewage yet remains, to a certain 

 extent, an open question, but it may be accepted as a universal 

 rule, that only under the most exceptionally favourable circum- 

 stances can the sale of it afford any adequate return upon the cost 

 of constructing sewage works ; so that, however profitable its use 



