This plan is suitable for crowded places, where a large amount of air is required in a short time. It is excellent for large schools, churches, and theatres. Its superiority for large elementary schools has been proved at Dundee by the experiments of Drs. Carnelley, Haldane, and Anderson, the results of which are summarised in the following table:—
| NO. OF SCHOOLS. | NO. OF ROOMS. | CUBIC FT. ALLOWED PER PERSON. | CARBONIC ACID IN 10,000 OF AIR. | MICRO-ORGANISMS PER LITRE | ||
|---|---|---|---|---|---|---|
| BACTERIA. | MOULDS. | |||||
| Mechanical ventilation by warmed air | 6 | 32 | 160 | 12·3 | 17·5 | 1·0 |
| Natural ventilation andhot pipes | 17 | 43 | 176 | 16·3 | 96·5 | 1·1 |
| Natural ventilation and open fires | 33 | 84 | 145 | 19·2 | 153·2 | 4·8 |
The air to be admitted may be warmed by passing it over hot-water or steam-pipes. In large establishments, as in hospitals, theatres, etc., it has been arranged so that the incoming air is passed through a screen of coarse cloth, which is kept wet by water trickling down each cord. The air is thus kept moist and freed from dust.
The great advantage of the plan of propulsion, is its certainty. By it the temperature, moisture, and freedom from suspended matters of the incoming air can be exactly regulated and controlled. Its chief disadvantages are that (1) it is somewhat costly, and (2) the apparatus requires skilled supervision. On the other hand it maintains the air in crowded rooms in a condition which cannot be secured by any other method. When combined, as is done in the Houses of Parliament, with the use of a flue for the extraction of foul air, this plan answers admirably.
The Relative Value of Artificial and Natural Ventilation scarcely needs to be discussed. They are both valuable, but under different circumstances. In dwelling-rooms natural ventilation by doors, windows and chimney usually suffices, especially if the products of combustion of gas are removed through a special flue. Natural ventilation is always occurring, and only needs a little aid in domestic life. For large rooms occupied by many persons artificial ventilation is necessary to maintain pure air.
Whatever method of ventilation is adopted, the atmosphere will remain to some extent polluted, if the room and its occupants are dirty. In certain experiments made by Carnelley in schools, it was found that dirty children increased the number of micro-organisms per litre of air more rapidly than dirty rooms. Thus:—
| DEGREE OF CLEANLINESS OF | CLEAN. | MEDIUM. | DIRTY. |
|---|---|---|---|
| Children | 63 | 99 | 159 |
| Rooms | 85 | 94 | 139 |
Number of micro-organisms per litre of air.
Hence cleanliness of rooms and of their occupants is quite as important as a good system of ventilation.