The second method, that of broad irrigation, is one that finds considerable favour with a large number of engineers and agriculturists. The great sewage-disposal cry has always been, “Put back on the land what you have taken from it, or some day there will be no beef and no bread.” The difficulty is to always find land in sufficient quantity and so situated as to be available for this purpose. Almost any soil is, however, suitable for irrigation, provided it is well and properly drained. The quantity of sewage which should be used for this purpose per acre of land varies considerably, as will be seen on reference to a table prepared by Mr. Henry Robinson,[188] where the number of inhabitants to each acre irrigated is in one case (Leamington) stated to be 55, in another (Blackburn) 208,[189] the average being 137, the number of gallons per head of population per diem being 38. It may be useful to state here that a hundred tons of sewage will cover an acre of land 1 inch in depth, and that the value of sewage as a manure is said to vary from ¹⁄₂_d._ to 2_d._ per ton, or, calculated in another manner, about 10_d._ per head of population per annum. The best crops for a sewage farm (in addition to nearly all kinds of market-garden produce) are rye-grass, mangolds, beetroot, cabbages, carrots, potatoes, turnips, rabi, parsnips, lucerne, beans, wheat, oats, and barley; the cereals, however, are apt to run rather to straw, and some care is also necessary not to oversewage potatoes and some other root crops. Grazing cattle or cows can also be carried on with advantage, the presence of sewage having no effect either on the milk or flesh of animals fed on sewage farms. The third method, that of intermittent downward filtration, is really irrigation of land to such an extent as the land will filter or purify the sewage, the effluent passing off pure, irrespective of any effect upon the crops which may be growing upon the land. Great discussions have arisen (the principal battle-ground being Merthyr Tydvil) as to the maximum quantity of sewage which an acre of properly prepared land will treat, some of the champions of this system contending that a good porous soil properly drained to a depth of six feet will purify the sewage of 6000 persons per acre, others that only the sewage of 250 persons can be so treated.[190] There can be no doubt that earth has a most powerful deodorising power. Laboratory experiments have shown that as much as eight gallons of sewage can be filtered through a cubic yard of loamy soil in twenty-four hours, the soil being drained at a depth of six feet, the effluent therefrom having obtained a wonderful degree of purity. Much, however, must depend upon the character of the soil of the filtering area and the strength of the sewage which is being operated upon. The following description of the manner in which the earth acts upon sewage will be of interest: