In London and the majority of large towns, Bristol being the most notable exception, the ventilation of the sewers is effected wholly or in great part by gratings in the roadway, under which trays of charcoal were suspended (with the view of absorbing noxious gases) until their uselessness was recognized.

There is no denying that these grids occasionally emit offensive odours, but their closure would only lead to the scwer-gases forcing an exit through traps or gullies, perhaps into the very houses. Moreover, the air of a well-ventilated sewer is nearly odourless; offensive gases are in all cases the result of putrefying deposits, and their occurrence calls for the removal of the deposits, or of the structural defects causing them. Should smells be given out from the gullies in the gutters, as not infrequently happens during prolonged droughts, all that is required is to pour in a few pailfuls of water to restore the seal of the trap.

Country-Hojuses. - In the country nothing contributes so much to the picturesque appearance and enjoyment of a house, as well-grown and judiciouslyplanted trees and shrubs. They should not actually overhang the dwelling, but should be so placed as to shelter it from cold winds and heavy rains, as Well as from excessive heat. Conifers and evergreens best serve the former purpose, and deciduous trees, since they do not obscure the light in winter, the latter. Deciduous trees should not, however, be too near the house, as the fallen leaves foster dampness in the soil around; evergreens, on the other hand, shed their leaves less heavily, and in the drier months of early summer. As to the influence of woods and forests, it is only Decenary to add that, provided they are not so near as to impede the movements of the air around the house, they can have no ill effects, while they may afford protection against cold winds; and pine-woods are permissible in the closest proximity, possessing many advantages peculiar to themselves, with few or none of the usual drawbacks.

Plant- obtain from the carbonic dioxide of the atmosphere all the carbon mired for building up woody fibre and forming starch and sugar, the oxygen evolved in the fixation of the carbon - a process carried on in the green cells under the influence of light - more than compensating for the converse changes involved in their respiration properly so-called; the net result is the purification of the air for the maintenance of animal life.

Sites near rivers are as a rule undesirable for permanent residence, on account of dampness of soil, exposure to mists, and - when the river winds through deep valleys between steep hills or cliffs - want of free movement of the air and perhaps also of sunshine, while all river-side sites are exposed to the risk of floods. The dampness of the soil depends on the elevation above the level of the water, and if this be 15 feet or over the objection disappears. If less, the whole space on which the house stands must be covered with concrete or asphalt, and the walls constructed in such a manner as to be really damp-proof. Should the soil be pervious sand or gravel, it would be wise to build the house on arches, to have the basement open along the greater part of at least two opposite sides, in addition to the other precautions. Water-side houses should not be nestled in trees, and when built in a valley, the sunnier side of the valley should be chosen. The foot of a hill with a northerly aspect would be the worst possible site for a dwelling, especially if in a bend of the valley shutting out the morning and evening sun.

The health-giving influence of sea-air, whether due to the presence of ozone and the absenee of genus, or to factors at present unknown, is beyond doubt, but the site of a seaside house should be beyond the reach of or sheltered from the direct impact of the spray, and the outer surface of the walls should be rendered impervious, since they are apt to become saturated with salt, which keeps them constantly damp; the foot of a cliff is always unhealthy, as are the low alluvial whores of an estuary where the fresh and salt water meet, and the retiring tide leaves en expanse of mod.

Nuisances. - Trades and manufactures from which noxious gases are given off may be injurious to the health of the surrounding population, Of may be merely offensive to the sense of smell. They are alike the subjects of legislation, hut too often (the enforcement of the Acta being intrusted to local authorities in which the manufacturers have frequently a preponderating inflnenee) the law becomes practically a dead letter; besides, there is no legal or authoritative definition of what constitutes in each case "the best available means" of obviating the nuisance. Many trades are injurious to the health of the operatives themselves, but not to that of the neighbourhood; with these we are not here concerned. Those that arc public dangers are chiefly of a chemical kind. Hydrochloric acid is emitted from alkali-works and also from potteries during the process of salt-glazing. Chlorine is largely produced in bleaching-works and in the manufacture of bleaching-powder, the so-called chloride lime. In the roasting of certain ores, consisting for the most part of sulphides of copper and lead, sulphurous acid and arsenical vapours are passed into the air, and in the emanations from vitriol-works nitric arid is also present The latter is also evolved in the production of aniline colours, while sulphuretted hydrogen and ammonium sulphide are emitted from gas-works, and in the manufacture of sulphate of ammonia and other chemicals, and in enormous quantities from the spontaneous decomposition of alkali-waste. All these gases arc most injurious to health when inhaled in the concentrated forms in which they are met with in chemical and alkali works, potteries, and many other industries in which lead, arsenic, mercury, etc, are employed, but the consideration of these belongs rather to factory and industrial hygiene, which is beyond the scope of this work. In the small quantities in which they are present in the general atmosphere their injurious effects are most evident in the destruction of vegetable life around the great centres of industry, although they cannot but exert a deleterious influence on the health of the population. Such acid gases and vapours in the air act as irritants on the mucous membranes, of the respiratory passages, and to them as well as to the presence of sulphuric acid from the combustion of coal and gas in towns, is doubtle due the greatly increased prevalence of late years of naso-pharyngeal catarrhs and polypi The sulphides of hydrogen and of ammonium tend to impoverishment of the blood, and arsenical vapours to catarrhs of the eyes, bronchi, and the alimentary tracts, as well as to irritation of the skin, but these are rarely experienced outside factories, except as a result of arsenical pigments in wall-papers and other decorations, such as artificial flowers.

Among trades which, though not proved to have any deleterious effect on the health of the neighbourhood, are nuisances in consequence of the offensive odours they emit, may be enumerated those of the tanner, fell-monger, gut-scraper, blood-boiler, glue-manufacturer, tripe-boiler, soap-maker, tallow-boiler, fish-manure and superphosphate manufacturer, chemical manure-works of all kinds, and fried-fish shops. Brick-fields, too, are in some places nuisances from the smells given off by the refuse used to burn the clay, and cement-works, though usually isolated and in the open country, evolve highly acrid fumes. The smoke emitted from factory and other chimneys may be a very real nuisance, though of a different kind.

Sewage-farms on unsuitable soils, or under careless or unscientific management, often constitute grave nuisances, but it is quite certain that sewage-farms, properly designed and managed, may be absolutely inoffensive. They are so at Berlin, where asylums, schools, and gentlemen's houses have been erected evervwhere in the vicinity. Since, however, there can be no assurance that a sewage-farm will never be mismanaged, and since it is extremely difficult under certain conditions of wind and weather for even the best manager to prevent objectionable sights and smells, the house-hunter will act wisely in avoiding the too close proximity of such an outlet for putrescent matters.