A perusal of Chapter II. will indicate some situations in which “boils” may be expected. The conditions under which they may appear are so numerous that it may be stated they generally occur in any situation when a layer of loose soil has a superimposed bed of more or less impervious boil upon it which is perforated by the excavation for a dock or other work; and especially should a water-bearing stratum, such as chalk, underlie a sand stratum; also when a water-bearing stratum is superimposed upon sand which lies upon an impervious bed such as clay, as water will percolate to the sand; and when sand overlies a water-charged stratum; or at the outcrop of chalk hills near the site; or where sand is below an impervious layer of clay and the latter is tapped. The “boils,” of course, become more serious as the head of water increases, and also when the strata dip towards the site. An impervious bed may be of such thickness, that when the lowest foundation is excavated there may be no fear of water from any underground source being forced up through it; however, unless the thickness of the upper layer is known to be nearly uniform over the site of a dock, which is seldom the case, the water pressure, aided by the weight of the structure, may separate or loosen the soil at a weak place, and then a “boil” will be the result. The thickness of the crust required, which may be anything from about 15 feet upward, may be approximately ascertained by weighting the earth considerably above the load it will permanently have to sustain, and by watching the subsidence and general effect. Great care should be taken not to perforate the firm stratum, as danger will at once ensue should loose soil, such as sand, be tapped, for cavities will then be produced causing subsidence, probable fracture of the firm stratum, “boils,” and slips of serious extent. When a heavy structure has to be built upon such a soil, and it is impossible because of the expense to place the foundations except upon this superficial bed, experiments in the direction indicated should always be made, and any other means of proof that circumstances may allow other than by perforation of the stratum: in any case every effort should be made to reliably ascertain that the firm stratum is of uniform depth and character.