(anc. Venusia), a town of Naples, in the province of Basilicata, in a small plain, 23 miles N.N.E. of Potenza. It has a ruined castle of the fifteenth century, a splendid cathedral, several parish churches, five convents, an hospital, almshouses, an exchange, and a fine Norman abbey, with a church containing many interesting monuments. The ancient Venusia was a flourishing Roman colony; but it is chiefly remarkable as the birthplace of Horace, b.c. 63. Pop. 6000.
VENTILATION is the art of providing any confined area or apartment with an adequate supply of air in a condition suitable to the purposes for which it may be required. The air is usually introduced in a stream which maintains a freshness and purity of atmosphere in the place that is ventilated, any contaminated air being removed as the fresh air enters.
The progress of science has shown that no more prolific cause of disease and death exists among men than a vitiated and ill-regulated atmosphere in their habitations, whether it operate slowly and insidiously, producing scrofula, consumption, dropsies, and other complaints; or, with sudden and extreme fatality, as in the Black Hole of Calcutta, in mines and wells loaded with carbonic acid, or in ships where the hatches have been battened down during storms. In a case that occurred a few years ago in the Irish Channel, every one below, about seventy altogether, was suffocated. Such examples, and the ever-active power of air during life, have at last established the conviction that much more attention ought to be given to this question than has usually been accorded to it, and that ventilation ought to be a primary instead of a secondary object in all architectural structures. It can never be so satisfactorily adjusted when it is merely introduced into buildings erected previously without any adequate provision for it. In numerous operations of nature and art, and more especially during combustion, respiration, and the decay that attends the putrefaction of animal and vegetable matter, unwholesome products are evolved; and so deleterious are these to animal life, that death is the necessary consequence when they accumulate in a concentrated form around the person. In a smaller proportion, they produce an endless variety of discomfort and disease, from a slight sense of languor or debility to the most violent apoplectic seizures, suppressing for a time all attempts at exertion either of mind or body; while, on other occasions, they gradually undermine the constitution, and induce a permanent loss of health. In extreme cases death is instantaneous or less rapidly, according as the oxygen of the atmosphere is withdrawn, or an impregnation of poisonous gases communicated to it.
The air of respiration is the material agent that sustains and harmonizes all the physical changes in the human frame, supplying the important element that oxidizes the blood, consumes much waste animal matter, and contributes essentially to that organic chemistry by which heat and electricity, as well as all special products, are developed in the living system. In the external atmosphere, air is constantly supplied to the bodies of animals, flowing to them on every side, and displacing the vitiated air which they produce. All habitations and other structures, whether large or small, for the use of man and other animals, must be provided with apertures for the ingress and egress of air, and additional means, when necessary, to secure the required supply. Without this they are deprived of the natural ventilation that is essential to life, and for which no other substitute can be given. We draw upon the atmosphere no less than 1200 times an hour, on an average, for nourishment and support, during the whole period of our existence; we consume oxygen and replace it by carbonic acid; and were air withdrawn from us, or changed much in its qualities, death would inevitably ensue. The air acts incessantly, not only on the blood as it passes through the lungs, but also on the surface of the body; and disease and death may arise from an unwholesome atmosphere in contact with the skin, even when the lungs are supplied with pure air. Well attested cases are recorded where severe oppression has attended the action of an amount of impurity so small as from the tenth to the seventh part of sulphured hydrogen gas, while the absolute amount of impurity in air tainted by miasma is so excessively small, that its precise nature, as well as the minuteness of its weight in the most pestilential atmospheres, is unknown.
The supply of a fresh and wholesome atmosphere may accordingly be ranked among the first and most essential necessaries of life. In the atmospheric ocean which rests on the surface of the terraqueous globe, a perpetual movement, or rather translation, is maintained on a great scale by numerous causes more especially, however, by the unequal action of the rays of the sun on the equator, and at the poles; the colder air moves along the surface from the poles to the equator, while the warmer air from the equator ascends and proceeds in a contrary direction towards the poles. These great and primary currents are modified in endless variety by the attraction of the sun and moon, the rotation of the earth, the relative effect of land and water, the ever-varying influence of local temperatures, volcanic action, meteorological phenomena, particularly the evaporation and deposition of moisture, the electrical condition of the air and of the surface of the earth, and the innumerable changes that attend chemical action in the mineral kingdom, as well as in those that occur in the organic world. The animal and vegetable kingdoms not only contribute to the movement of the air, but are the great causes of the most important changes induced in it, and the means of preserving the unity of its composition. The animal kingdom consumes its oxygen and produces carbonic acid, while, in the vegetable kingdom, the great tendency is to absorb carbonic acid and replace oxygen. But wherever these great movements are interrupted by local causes, or an undue accumulation of vegetable and animal debris takes place, there the right balance is not sustained; pestilential effluvia contaminate the air; and were it not for the wind, the rain, and the impetuous storms which, from time to time, visit such localities, and the operation of a peculiar diffusive power, in consequence of which no gas can accumulate permanently to the exclusion of other gases on the surface of the earth, whatever may be its specific gravity, they would at last become fatal as the plague of death in Java, or the carbonic acid springs in Bavaria, in the Vallo Del Lago in Italy, and other places notorious for their destructive atmospheres.
In selecting a site for a city, a house, or any establishment where numbers are crowded together, too much attention cannot be paid to its natural ventilation. An aspect towards the south, a dry gravelly soil, a moderate elevation securing efficient drainage and freedom of access to the air, with protections from offensive currents, and an immunity from local impurities, are great desiderata.
In the open air the temperature is generally under that of the body; and the air expired from the lungs, and also that in contact with the surface of the body, being expanded by the heat which it receives, escapes from its levity, and a fresh stream is immediately supplied; but when the body is confined within a limited space, special arrangements must be made to admit of a constant renewal of fresh air.
This ventilation must be modified and adapted to the circumstances of each individual case, and the state of the external air. It is a subject, therefore, of great extent, and as various in its details as the climate in which we live, the habitations in which we dwell, the occupations in which we are engaged, the food on which we live, the means of protection against heat and cold, and the peculiarity of constitutions which each may have. We shall therefore content ourselves in the space allotted to this article, by pointing out the leading facts connected with the more prominent objects which it includes in reference to the frame of man. It must not, however, be forgotten that this branch of science is as yet so imperfectly attended to, that it is impossible to turn to any city without seeing much discomfort, disease, and even death, induced from time to time by ignorance of the laws of ventilation; and, in assemblies of every variety, the whole audience is not only too frequently subjected to extreme uneasiness, but the tone of the mental faculties, and the capacity for exertion and attention, are also affected by the state of the atmosphere.
I. SOURCE OF AIR.—This cannot be too particularly examined. Many buildings are supplied with air from a low level, where the ground is too frequently left in a neglected condition, and exposed continuously to different sources of impurity. At night, in clear weather especially, when the cold produced by radiation from the earth is great, and more or less at all times, cold air is too often supplied from the surface of a street loaded with the offensive emanations which they evolve, and these become still more disagreeable when subjected to the action of heat in a warming apparatus. By taking air from the highest attainable point, avoiding the immediate vicinity of chimneys, a much better atmosphere is secured, and it may be conducted downwards to any required depth by a flue. In extensive public buildings, situated in localities noted for the impurity of their atmosphere, the erection of air-towers to draw down a comparatively wholesome atmosphere is an object of great importance. An elevation of five, ten, or twenty feet often gives much better atmosphere. In all lofty buildings, air ought to be supplied, when requisite, from above; and where local circumstances render it desirable, air is sometimes advantageously introduced by turrets from an altitude of one, two, or three hundred feet. When air is pure, and dry cool vaults are available, they are invaluable in cooling it during hot weather. In large towns, such as London and Manchester, where local impurities abound, and particularly when fog and frost are observed at the same time, special means may be adopted for removing those that are most offensive. Filters have been constructed for air on the same principle as for water. Few of the more simple arrangements of this kind, that have been sustained for any length of time, have been found more serviceable than filtration through any porous texture to exclude suspended blacks (soot). Washing them also has, in some cases, been tried, by moistening the filter with a stream of water. Lime-water is preferred where sulphurous acid, carbonic acid, hydrochloric, or hydrosulphuric acid are present from manufacturing operations in any notable proportions. Perforated zinc, or porous gauze, steeped previously in a solution of hydrochlorate of zinc, have been used largely for the purification of air.
II. INGRESS AND EXGRESS OF AIR.—Though windows, especially when made to open above, afford a very convenient means for the ingress and egress of air, they should not be the sole resources for ventilation in any apartment in new buildings, especially where gas-lights, or any other powerful and brilliant lamps are used, or where they are likely to be crowded with visitors. An aperture in the ceiling of every apartment, constantly drawing off vitiated air, is the greatest desideratum. It should communicate with a flue discharging into the external air, and be protected therefrom local draughts, or down-draughts, arising from the action of the wind. Valves should regulate with precision the amount of discharge, and arrest it entirely when they are closed. An aperture near the ceiling in an ordinary chimney is sometimes sufficient, but it cannot always be trusted. An independent discharge is better.
The ingress of air should be equally under control, and supply air where it can enter gently, and diffuse itself most widely, without local draughts, through the apartment to be ventilated.
A very important arrangement has been introduced in numerous buildings, which consists in providing a large aperture above all internal doors leading to individual rooms, while valves regulate the amount of clear opening permitted. The staircase and passages being constantly supplied with a mild atmosphere, such rooms can always be ventilated from this source, the aperture admitting fresh air and discharging vitiated air by the general discharge from the passages and staircases. There, too, the principal ingress, as well as the channel for the escape of vitiated air, is constructed so that the cold air, as it enters, receives indirectly some warmth from the vitiated air. In large and public, and where apartments are crowded, it is desirable to maintain a more perfect separation between the fresh and the vitiated air, so as to facilitate the ingress of the former, and the escape of the latter.
Where a more complete system of ventilation is desired, nothing conduces more to secure this object; in individual habitations than the construction of a special flue for the discharge of vitiated air. A connection can be established at any time between it and individual rooms, closets, or cellars, and all vitiated air expelled with a velocity proportional to the amount of fuel consumed in this flue.
Thus, then, in the habitations of the people, the independent warming and ventilation of each individual room is the first object; whatever system of heating be employed. The warming of the whole house, to a certain extent at least, through the hall, passage, or staircase, is another desideratum. The special action of a ventilating flue, available at all times and seasons on one or more apertures, and rushing in a well defined but partial stream, so as to produce local currents, may be exceedingly offensive, while, if divided into innumerable smaller streams, by causing it to pass through a porous texture having the most extensive possible surface, it may proceed so gently as not to be perceived. Air ought always to be admitted in this manner, or at such a distance from those upon whom it is to act, that its impulse may be greatly moderated before it reaches them. Wherever a proper supply of air is admitted, this equalization is essential, more particularly in crowded apartments; and the greater the degree to which it is carried, the more perfect and agreeable is the result. Nothing is more common than to see apartments ventilated effectually, so far as may be necessary for the removal of foul air, but with a movement that induces a most offensive series of chilling draughts, if means be not taken to warm the supply of air given, and for introducing an effective equalization. In rooms for invalids, this subject becomes of great consequence, especially in diseases of the chest. To a great number of constitutions, unequal currents are as dangerous and offensive as an oppressive atmosphere.
IV. AMOUNT OF AIR NECESSARY FOR VENTILATION.—Few subjects present a greater diversity in practice than the amount of air given for ventilation. From 2 to 4 cubic feet per minute for each person have usually been considered a large supply, but this is far beyond the amount commonly allowed. A more slight examination, however, will show that even this amount is often too small. If the process of respiration be accurately examined, it will be observed that a cubic foot of air, or more, is involved or mixed and contaminated with the air discharged from the lungs, at each expiration, independently of that affected by the skin. Such a supply, therefore, is at least desirable, were the air always at a mild and genial temperature. According to Dr D. B. Reid's experiments, where the effects of variable quantities of air were tried upon numbers included in an experimental apartment, not less than 10 cubic feet of air per minute should always be allowed when it is warm; and to sustain the atmosphere in all its freshness and purity, even a much larger quantity is at times desirable. At the late houses of parliament, from 36,000 to 50,000 cubic feet per minute have occasionally been given in sultry weather in the House of Commons alone, or about 60 feet per minute to each individual in a crowded house. This question of the amount of air necessary for ventilation, taking the deterioration produced by the human frame along into account, is often complicated and influenced by a vast variety of circumstances, of which the following are the most important. In the preceding and following remarks, a temperature of 65 degrees may be considered as the average most generally desired when there is a steady but uniform and gentle movement in the air. It is not so much the amount of supply with which life can be sustained that is the question, as that which it may be desirable to afford so as to keep the system in a state of vigour and comfort, even when the air is much warmed.
1. The Purity of the Air supplied.—In general, the less pure the air the greater the amount necessary for ventilation, especially if it be loaded with moisture, and charged with offensive exhalations from the lungs and skin. But rare cases occur, when the atmosphere is so largely charged with external poisonous effluvia, that it becomes an object to use as little as possible to avoid this source of contamination. In one instance, cases of fever are reported to have occurred at a particular period in every room that was largely supplied with air, while, in other rooms in the same building, with a much less supply of air, no disease was noticed.
2. The Temperature of the Air.—This is the most important circumstance affecting the supply necessary for ventilation. When the air is very cold, and the moisture of the breath is condensed in hoar-frost as fast as it escapes from the lungs, a proportion of air extremely small compared with the usual allowance desirable will be sufficient for ventilation. Air, under ordinary circumstances, is below the temperature of the body; it therefore acts as a cooling power. But the higher its temperature, and the more nearly it approaches that of the body, the larger is the quantity required to produce an equivalent cooling power. Further, in warm weather, it is charged with more moisture than in the cold season, while the body is at the same time more exhaling of moisture. These and other circumstances tend to render the supply of air desirable in warm weather far greater than the mere arithmetical increase in the temperature would, at first sight, appear to demand. But here we must advert to that popular error that the temperature, as indicated by the thermometer, is a proper guide to the quality of air in respect to the warmth that may be most agreeable. The temperature, as a moment's reflection will show, is a very imperfect guide, unless the velocity of movement and chemical qualities of the air, especially in reference to moisture, be also taken into consideration. A small quantity of air, stagnant, and at 32° Fahr., does not cool the body more than a larger supply at 40°, 50°, 60°, 70°, 80°, or 90°, and, indeed, at any temperature below that of the living system, if brought to act upon it in sufficient proportion. A large quantity of air not so cold may be made to produce the same amount of cooling effect as a less amount of colder air.
3. Moisture in the Air.—Air in winter usually requires the addition of moisture when warmed and introduced into any apartment, as at this period it has deposited a large proportion of the moisture associated with it in the air. When the air is warmed by the approach of summer, it gains moisture from the surface of the earth or of the ocean, and thus acquires more of the pleasing and agreeable qualities which a summer atmosphere presents; but if warmed and introduced into any apartment without the previous addition of moisture, then, having had its power of action upon moisture increased, without receiving a corresponding supply, it absorbs moisture with extreme rapidity from the surface of the body and of the lungs. By taking away an undue proportion, it produces a harsh and disagreeable impression. The injection of steam into the air, or the evaporation of water from shallow pans placed over the heating apparatus, removes the defect.
Moisture is frequently communicated to the air with the view of cooling it by the reduction of temperature attending evaporation.
In some climates, again, a redundancy of moisture forms one of the greatest sources of oppression, and is regarded as one of the most powerful causes in developing the activity of miasma.
The influence of different degrees of moisture in the air is as various in different constitutions as that of different temperatures. Again, some individuals exhale moisture almost solely by the lungs, while in others the skin is equally active. According to the relative condition of the living system, the air, in reference to moisture, may either exhale or absorb moisture from it. A large quantity of air charged with much moisture, but not saturated, may produce as much evaporation from the body as a less quantity of air containing little moisture in solution.
4. Disposition and Habit.—A very wide range of experiments has shown that the constitutional peculiarities of different individuals vary as much in respect to the amount of air desired, and the temperature at which it is preferred, as in respect to food or drink. In the British Houses of Parliament, no temperature below 52° or above 76° has been demanded for a series of years. It is often very difficult to determine, however, how much is due to absolute peculiarities of constitution, and what is dependent on the circumstances of the moment, more especially the state of occupation or excitement, the time that has elapsed since any refreshment may have been taken, and the nature and quality of the repast, the clothing in use, the previous exposure, the temperature, moisture, and other circumstances affecting the quality of the air. The brilliancy of the illumination also affects the supply of air required by some constitutions. The force of habit is nowhere exemplified in more marked manner than in the amount of contamination which different individuals may bear. Some pass habitually much of their time in air in which a candle does not burn, though a lamp may be maintained in combustion in the same atmosphere. In public buildings, the great object is to sustain a uniform standard of ventilation suitable to the great majority of the audience, and to reject the complaints of individuals with extreme constitutions in regulating this standard. It is not unreasonable, however, to provide a judge who is confined for a long period to a particular place in a court of law, and others in parallel positions, with more power in varying the atmosphere in their vicinity than is given in any other part of the court.
V. Nature of Heating Power.—This is necessarily as various as the climate, the fuel available, the building to be heated, and the precise effect desired. Few circumstances require more care in connection with ventilation. The open fire, so much prized from its lively and cheerful appearance, exerts also an agreeable effect upon the animal system by the light radiated along with the heat, and the movement of air which it sustains as it ventilates the apartment in which it is placed; but in point of economy of fuel, or facility of regulation, so as to maintain an equal temperature in large apartments, it is perhaps the least desirable of all kinds of heating apparatus. The great beauty, therefore, which its appearance presents, the absolute purity of the heat which it conveys by radiation, and the extreme facility of access which it offers for important for many different purposes, ought to be contrasted with the attendance which it requires, the dust and ashes which it leaves, and the tendency, when neglected, to produce back-smoke, if the action of the flue be not maintained with proper force. In connection with ventilation, the following points require special attention in the construction of the common fireplace. 1. It should be provided with an independent supply of air entering in its immediate vicinity, to be employed when heat is required in any apartment without changing much air there, as in warming the apartment before it is occupied, or moderating offensive currents near the fireplace. 2. An open fireplace, unless the air enters at or near the ceiling, often produces little or no ventilation above the level of the chimneypiece, and even then it does not afford the best and purest atmosphere. 3. The air above may be comparatively stagnant, and offensive in the extreme from lights and the products of respiration, while a fresh current moves along the floor to the fireplace alone, if no separate discharge be provided above. 4. The introduction of a valve to regulate or diminish the excessive draught of some chimneys, often adds more to the comfort of the rooms on which they act than any other measure. Sometimes a reduction in the escape at the top, even to a quarter of the aperture previously given, is found to be a great improvement both in point of comfort and in the economy of fuel.
Of other arrangements for heating, the mild hot-water apparatus, where the water is always under the boiling temperature, affords a very perfect and equal diffusion of heat, when properly arranged,—a point of great importance in producing equality of ventilation. Stoves presenting an extensive surface at a moderate temperature, varying from 100° to 200° degrees, as the Russian, Prussian, and Swedish porcelain stoves, or Dr Arnott's iron stove, come next in order. Those varieties should be preferred which are provided with chimneys, both for the ingress and egress of air, that have no direct communication with the apartment in which they are placed; otherwise, when worked most economically, they are all liable to evolve gases injurious to health. If they are not adjusted by persons who thoroughly understand their action. Stoves and other apparatus, where the iron is heated to a high temperature, may be in many cases more economical than the preceding variety; but from the manner in which they affect the air, they are not so conducive to health, and greatly interfere with regularity of ventilation, the very hot air from them ascending rapidly to the ceiling, while a cold atmosphere, almost unaltered, is often left below. The stoves and heaters usually employed for conveying warm air to the principal apartments and passages of houses in very cold countries, generally heat too much, and are provided with channels that are too small, while they are deficient in arrangements for returning air from the passages and individual apartments, when there is little or no ventilation of the air supplied.
There are three different kinds of steam-apparatus.—1. Mr Gould's low-temperature steam-apparatus (a recent patent). A variable proportion of steam is admitted into thin but broad iron cases, part of the air only being expelled, according to the temperature required. A somewhat similar effect may be secured by surrounding steam-pipes at 212°, with discs or leaves of zinc or other metals.
2. The ordinary steam-apparatus, in which steam is used at the temperature of 212°.
3. Perkins's high-pressure steam-apparatus, in which a more elevated temperature is maintained, according to the pressure on the steam.
With such a variety of resources, great facilities are afforded in regulating the ventilation of public buildings. The more certainly the floor is warmed, the milder the source of heat; the more universal the diffusion of the entering air, the more certain the continuous escape of vitiated air; and the means of altering a valve to adapt the amount of air to the numbers present, the more satisfactory will the ventilation be.
In buildings for public assemblies, subject to great and sudden fluctuations of attendance, hot and cold air-chambers are provided, from which any supply of warm or cold air may be obtained. A mixing-chamber, for mingling the various proportions that may be supplied is a great additional convenience.
VI. Source of Movement.—An alteration of specific gravity in the air, dependent on an alteration of temperature produced by expired air, and the general warmth of the body, is the great, the more natural, and the most economical source of movement, under all ordinary circumstances. When the openings for ingress and egress are well arranged, any ventilated apartment may be compared to a piece of apparatus in which the current of air, entering rapidly by a narrow channel, expands with the greatest possible diffusion into a slow-moving stream, occupying the principal portion of the area of the place to be ventilated, and gathering together again in a smaller channel by which it escapes, where the velocity of movement increases as the area diminishes.
In ordinary circumstances, where nothing more can be afforded, two openings at different levels will always give much relief, the one for admitting cold, and the other, which should be as high as possible, discharging hot and foul air. Where one opening only is made for ventilation, one part of it admitting and the other discharging air, the nearer the ceiling it is placed the more effectually does it act. A crowded room, in which fresh air enters on every side with the most gentle movement and at a proper temperature, so that its impetus is not perceived, vitiated air escaping in a central stream, and all products from artificial lights being carried away by the same current, presents a perfect system of ordinary ventilation. It is presumed that double glazing is introduced to prevent down-draughts from ice-cold glass.
In public buildings, where long sittings are held, under every variety of circumstance, in different seasons, with ever-varying numbers by day and by night, and amidst endless changes of the external atmosphere, it becomes impossible to regulate the ventilation satisfactorily without a power to move the air, and without appropriate valves. For this purpose ventilating chimneys or shafts, worked by fire, fanners, pumps, or screws, driven by a steam-engine or water-wheel, a jet of steam, a stream of air acting as a blow-pipe, a current of electricity, or any other power, may be used. The shaft and the fanner are usually preferred, especially the former.
The moving power of air itself is constantly used both for forcing and exhausting air. The windmill and the cowl are familiar illustrations where this power is resorted to, but the continuity of its action cannot be depended on. A chimney, a shaft, or any other external discharge of vitiated air, acts with increased power when it assumes a conical form at the top. Elaborate experiments on this subject show that this power is increased if a small part of the upper portion be inclined outwards.
A stream of water, descending in one or more tubes or flues, is often used as a ventilating power, especially where it is made to fall as a shower, in condensing poisonous ingredients or other impurities.
VII. VARIETIES OF VENTILATION.—The condition of the human frame being as various as the peculiarities of individual men, the habitations in which they dwell, the climate in which they live, and the circumstances under which they assemble for business or recreation, ventilation assumes a never-ending variety of aspect in detail, and in the simplicity or care required to insure a successful result. It is a matter of perfect indifference, in point of effect, whether air take an ascending or descending movement, or pass from side to side, descend and ascend, or move in any other compound manner, when an apartment contains only a few individuals compared with the number it can accommodate. If the air be supplied abundantly, of good quality, of an agreeable temperature, and all source of impurity be excluded, or rapidly removed, air may be made to move in any direction that may be required; and, in special cases, a lateral, or even a downward current, may be advisable, particularly in protecting works of art from the dust put in motion by a crowded peripatetic assembly, or in hospitals with incurable patients totally unable to take any care of themselves. But as a matter of economy, practice, and daily convenience, the application of heat at the lowest level, the ascent of vitiated air, and the supply of fresh air with the greatest diffusion practicable, are the most important objects in warming and ventilating all crowded apartments.
a. Natural Ventilation.—This term is used in all cases where the movement of the air arises spontaneously from a difference in specific gravity between the internal and external air. The following example illustrates this variety in its simplest form. With a single aperture, sufficiently large, in the ceiling of any apartment, and a free communication externally, cold air usually descends by one part of it, and warm vitiated air escapes by another. In improving such an arrangement, the descending air may be diffused by a porous curtain, or perforated zinc, so as to fall more widely and more gently, and the escape of the vitiated air may be facilitated by erecting a shaft or tube over the other portion. A current descending from one part of a ceiling, and escaping, when vitiated, by another, is not a satisfactory method of ventilation in crowded narrow buildings with high ceilings, especially in warm weather, as the air is apt, at times, to pass from one part of the ceiling to another. In very low rooms, this movement is not so objectionable, and sometimes affords a very valuable ventilation when the temperature is carefully adjusted.
Where force is employed to promote or insure ventilation, the three following varieties are usually distinguished,—viz., plenum, vacuum, and compound ventilation:
b. Plenum Ventilation is secured in all cases where air is forced into an apartment by a fanner, a screw, or a pump. The air within, in such cases, is slightly denser than the air without, and tends to leak outwards at all crevices of doors and windows. In some cases of disease, particularly asthma, air of great density is sometimes prepared by condensation with powerful machinery in special apartments, where it affords great relief to the suffering patient.
c. Vacuum Ventilation.—In this case a shaft or large chimney, or any mechanical power, is made to act as a pump, and draw out vitiated air. The air within the ventilated building has then less pressure than the external atmosphere, which tends to leak inwards at all crevices.
d. Compound Ventilation.—In this case, both plenum and vacuum ventilation may be said to be in operation, the vitiated air being extracted with as much power as that by which the fresh air is forced in. This is the most perfect form of ventilation for all very crowded assemblies, and prevents draughts at doors, either inwards or outwards, when in perfect operation.
During the last thirty years, more extended ventilating works have been constructed than at any former period. In these, the application of power to insure constant ventilation; a larger supply of air than had previously been considered necessary; a great increase in the magnitude of the channels of supply and discharge; a more careful attention to the temperature, moisture, and diffusion of the entering air; and the exclusion of impurities, externally or internally, whether from malaria without, lamps and candles within, or other sources, have formed the leading objects in actual practice.
In ships, a small steam-engine, or a power from a larger engine acting on a fanner, usually gives the ventilating force where artificial power is used; while tubes extending fore and aft in the hold draw vitiated air from every cavity above and below, and discharge it either externally or below the furnace.
The laws of communication of heat, the electrical condition of the atmosphere, the indications of the thermometer, barometer, hygrometer, anemoscope, and anemometer, the preparation of medicated atmospheres for rooms, inhalation and fumigation, are all objects of great importance in connection with ventilation.
The Carbonometer is a small bent glass tube, containing some test of carbonic acid gas, as a little lime-water, with a bulbous expansion, in which it mingles freely with any given volume of air transmitted through it by the action of a syringe, or by the descent of water in a glass, or metallic apparatus connected with it. The amount of white precipitate (carbonate of lime) indicates the general amount of impurity from respiration, exhalation, lamps, candles, ordinary fuel used in heating apparatus, or from any other source of carbonic acid.
The reader who desires to extend his knowledge of this subject will find much information in parliamentary reports, particularly on the Ventilating and Acoustics Arrangements of the late House of Commons, and on the Ventilation of the new Houses of Parliament, including Dr Reid's Examination at the Bar of the House of Commons in 1832; the evidence taken at the arbitration in 1853, not yet presented to Parliament; and in Dr Reid's Address to the Professional Men of England, Scotland, and Ireland; in parliamentary reports on the Warming and Ventilation of Dwellings (1827); on the Ventilation of Mines; on Education, Manufactures; on the Improvement of Health in Towns and Populous Districts in England and Wales. On Schools, Drainage, Hospitals, Prisons, and Graveyards, the same subject is largely introduced. Dr Hale wrote the most valuable of the early treatises on ventilation. Tredgold on Warming and Ventilation, is a very important work. In Dr Arnold's, and in Hood's, Richardson's, Wyman's, and Bernan's treatises on ventilation, hot water apparatus, and kindred subjects, much interesting information is given. In Dr D. B. Reid's Illustrations of Ventilation; Outlines of the Ventilation of the late House of Commons; Diagrams of the Ventilation of St George's Hall and New Assize Courts, Liverpool; Ventilation in American Dwellings; and in his works on chemistry, and the lithographs of his lecture and experimental rooms at Edinburgh, the result of numerous experiments is given on the ventilation of public buildings, private dwellings, mines, ships, and manufactories.