166 president's address — section h. 



known as Sub-surface Irrigation, is employed to a lesser 

 extent, chiefly on small areas. 



Broad surface irrigation consists in distributing sewage 

 over land, so as to water and manure it, and render it 

 capable of producing crops of various kinds. There should 

 be very little surface preparation if the site and the soil are 

 suitable, the sewage being absorbed by the surface soil, and 

 retained until the water is given off by evaporation. The 

 sewage of 150 persons can be disposed of for every acre, but 

 frequently much greater quantities of sewage are disposed of 

 in this way. " Intermittent downward filtration," as des- 

 cribed by its author, Mr. Bailey Denton, M. Inst. C.E., 

 " consists not in crowding sewage continuously on porous 

 land, often adopted to get rid of sewage, but of concentrating 

 it at regular intervals on as few acres of land as will absorb 

 and cleanse it without preventing the production of vegeta- 

 tion. It was first applied in 1871 at Merthyr-Tydvill, one 

 acre of land being used for the sewage of 1100 persons. 

 The sewage is distributed by means of ridges and furrows so 

 as to reach the roots of plants laterally through the soil 

 without touching the leaves. The sewage is allowed to run 

 on the land for about six hours at a time, and there are 

 generally four plots, which are used in this way in succession 

 for a period of one year, the other three remaining fallow, so 

 that each plot has a three years' rest." There is no well- 

 defined line between the Irrigation and Filtration methods, the 

 main difference being one of intensity ; the intermittent filtra- 

 tion method requires a very porous soil, or thorough surface 

 preparation and under-drainage so as to prevent saturation. 

 It is generally most convenient to combine these methods on 

 the same farm, using the filtration areas as a safety-valve to 

 the farm. Examples of Sewage farms may be seen in 

 Adelaide, Sydney, and Christchurch. In England, sewage 

 farms exist at Birmingham, Croydon, Leamington, Oxford, 

 Bedford, Doncaster, &c. One of the finest sewage farms in 

 the world exists at Genvilliers, and receives a large portion 

 of the sewage of Paris. The whole of the sewage of Berlin 

 is pumped to sewage farms. In America there is a sewage 

 farm at Pulraan. The purity of the subsoil waters at the 

 Genvilliers farm is stated by Pasteur to be most satisfactory ; 

 there are five lines of drains, which surround the village of 

 Genvilliers, and discharge into the Seine, and the farm pro- 

 duces the finest vegetables and salads which are sold in 

 Paris; the health of the Commune leaves nothing to be 



