It is only in the country that one has a perfectly free choice as to the site of a house, including the several conditions of soil, elevation, and aspect.

Soil. - The influence of soil is more felt in the country than in towns, where paving and drainage tend to obliterate the differences due to greater or less permeability. Generally speaking, granite, slate, sandstone, and limestone rocks provide the very best sites; sands, gravels, and chalk are next in order of merit, provided they are deep and free from an admixture of clay; while marks, brick-earth, and clays are the worst, since they absorb but little of the rain, but hold it in every depression until it evaporates.

Surface accumulation, however, can always be prevented by paving and surface-drainage; ground-water is usually a more important point for consideration, and is, at the same time, far more difficult to deal with. No site can be considered healthy where the ground-water is found at less than 12 feet from the surface, or 15 feet if its level be subject to considerable fluctuations. If nearer than 15 feet its level should be. as far as possible, lowered and fixed by subsoil drainage; and the area on which the house stands should be covered with concrete or asphalt, to prevent the ascent into the building not only of moisture but of the ground-air, whether through the rise of the ground water, or the warmth of the house. Pervious soils, as chalk and gravel, if forming beds of 10 or 15 feet in thickness over an impervious bed such as clay, are among the most dangerous and deceptive of sites, since the ground-water will probably be found within a few feet of the basement, and may at times rise into it; and should there chance to be a depression in the clay at that spot, the house will practically stand over or in a concealed pond. In the case of a river-side house - if it cannot be at any considerable height above the river, - a clay soil, the stiffer the better, is far preferable to one of gravel, as the latter will be permeated by water, which will rise and fall with the river itself. The more permeable a soil the more liable it is, with its ground-water, to become polluted by soakage from cess-pits, dungyards, and foul ponds, - a danger which in the case of chalk is increased by the frequent occurrence of fissures of extreme length.

A good fall to assist surface-drainage as well as that of the house and subsoil is always desirable, but in building on a steep hillside several precautions are necessary. On no account ought a house to be built against a cliff, whether a natural one or formed by excavation: and if the hill rise abruptly within a short distance, a deep trench should be dug along it to receive and divert the surface water and storm-water from the site of the buildings; such a trench may, if made large enough and planted, add to the beauty of the grounds. Another site to be avoided is the Hue of contact on a declivity between a superposed pervious and a subjacent impervious bed, if the plane be not in an opposite direction to that of the surface, since the ground-water in the upper bed will emerge along this line in the form of springs, at any rate after heavy rains.

So, too, in building on clays, stains of gravel, which act as drains for surface-waters, and give rise to "land-springs", should be looked for and drained. A further objection to clays is their tendency to shrink during long periods of drought, thereby leading to sinking of foundations and cracking of walls.

Unless well protected by plantations of pines or other evergreen trees, houses on plains are exposed to cold winds; whereas in hilly, or even in undulating districts, it is not difficult to select sites that shall be sheltered from cold or stormy winds and yet enjoy the maximum of sunshine at all times of the year. But a further advantage sometimes obtainable in hilly countries is that, by certain arrangements of ranges of hills of even moderate altitude, rain and thunder-clouds are diverted, so that particular localities or valleys may enjoy an exemption from the heavy rainfalls general in the district, and a drier and warmer climate than their geographical position would lead one to expect. Cockermouth, Sidmouth, and other places already referred to, are examples in proof.

Fig. 717 represents the banks of a river, rising on one side as lofty cliffs with villas or cottages perched picturesquely on every ledge; aa is a bed of gravel or drift resting on the rock bb, which is capped on the right by a stratum of sandy soil cc. The gravel is traversed by a river d, and the ground-water stands at the same level, ee, as the stream. The house f is sufficiently raised above the ground-water to be dry and healthy, but G on the other hand is not; indeed should there be any considerable rise of the water, it must mount into the cellars. The houses H and I are each on the rock, but while h is well away from the cliff and the storm-waters descending the slope will be carried off by the trench l, the other house I is built against the cliff and will inevitably be damp and unhealthy, exposed not only to storm-waters but to the drainage from cc, which in wet weather may issue at c like a spring. The |>osition of the house K is healthy though bleak.

The most potent factor in the healthiness of a locality is the soil, and it a no less wise than witty saying of the great German hygienist, Pettenkofer, that " we do not go from home for change of air, hut for change of soil". The ceaseless movements of the air preclude any appreciable differences in its actual composition, but the most diverse climates may occur within a few miles, as in the familiar instances of Bast and West Brighton, or Bromley and Penge in South London. The influence of the soil on temperature, radiation, humidity, and evaporation is marked and manifold. The heavest fall of rain sinks deep into the chalk, leaving the ground as dry as before, while on clay the water lies slowly evaporating for days or weeks, rendering the atmosphere damp and cold. Granite, trap, mountain-limestone, slate, and marble, may for practical purposes be considered impervious. Sandstones and chalk are decidedly porous, and capable of absorbing a considerable amount of water, which displaces the air that filled the pores when the rock was dry. Water, however, not only sinks into pervious rocks and soils, but rises upwards in them by capillary attraction, and to a greater height the finer the structure and smaller the pores; on the other hand, it sinks more easily and rapidly into the coarser rocks and looser and coarser soils. The constituent particles of clay arc of extreme fineness, and in the dry state would be described as dust; hence the capacity of clay for pasting air or water through it is less than half that of loam, and perhaps one-hundredth that of sand and gravel

Fig. 717.   Some House sites.

Fig. 717. - Some House-sites.