These amounts are trivial, and if the water be supplied from a private well in the grounds it is evident that, allowing for evaporation, _we should pump from the subsoil rather more than we return to the surface_. Again, it must be remembered that the house with the greatest amount of slops has, as a rule, the largest curtilage. A mansion containing twenty persons with unlimited baths, laundry, and stables would not probably give more than 40 gallons per head, or 800 gallons a day, which is a trivial amount when considered in connection with a park of 20 or perhaps 200 and more acres. It is necessary to insist that the amount of slop-water to be dealt with in isolated houses is usually trivial in proportion to the land available for its purification. Tidy was of opinion that, employing intermittent downward filtration for the purification of previously precipitated crude sewage, an acre might be sufficient for 7,000 persons. This would give 1/1000 acre, or about 44 square feet, for a household of seven. At this rate my consulting room in London, which measures 24 × 18 = 432 square feet, would be an area large enough for nearly 70 persons. I think the estimate is too small; but even if one multiplies it by ten it is evident that the amount of land necessary for treating the domestic slop-water of a house is much smaller than might be supposed. In places where unlimited water is obtained by merely turning a tap these estimates are very liable to be exceeded, especially when those who turn a tap _on_ are too forgetful or lazy to turn it _off_. I feel certain that anyone who experiments on this matter as I have done will be simply astounded at the small amount of ground which is necessary. Critics of the plans which I have advocated, and am now about to advocate, sometimes hint that the whole curtilage of one's house must be unwholesomely sloppy. Such a statement shows a complete ignorance of the whole subject. Few, if any, of the writers of hygienic text-books seem to have really studied the life-history of slop-water, and it is matter for regret that in some of these books the chapters devoted to domestic hygiene deal more with patents than with principles, and are illustrated more by woodcuts culled from tradesmen's catalogues than by any practical knowledge possessed by the writer. PRINCIPLES OF SLOP-DRAINAGE If domestic slop water is to be dealt with successfully it is necessary— _1. That all waste pipes terminate well above the level of the ground._ This is a point too much neglected by architects, who are very prone to carefully put all waste pipes below ground level, so that any purification of slops without pumping is impossible.