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, anemometer, 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 Acoustic Arrangements of the late House of Commons, and on the Ventilation of the new House of Parliament, including Dr Reid's Examination at the Bar of the House of Commons in 1852; 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 (1857); 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 Ventilating, is a very important work. In Dr Arnott'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 Azize 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. (D. B. R.)