In =Ventilation by Propulsion= a fan is used as in the last plan, the air being propelled along conduits leading from it into the room to be ventilated. The size of the conduits being known, the amount of air to be discharged can be regulated by timing the rapidity of the revolutions of the fan. 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:— Column headings: A No. of Schools. B No. of Rooms. C Cubic ft. allowed per person. D Carbonic acid in 10,000 of air. E Micro-organisms per litre. F Bacteria. G Moulds. ┌───────────────────────┬─────┬──────┬───────┬────────┬──────────────┐ │ │ A. │ B. │ C. │ D. │ E. │ │ │ │ │ │ ├────────┬─────┤ │ │ │ │ │ │ F. │ G. │ ├───────────────────────┼─────┼──────┼───────┼────────┼────────┼─────┤ │_Mechanical ventilation│ │ │ │ │ │ │ │ by warmed air_ │ 6 │ 32 │ 160 │ 12·3 │ 17·5 │ 1·0 │ │_Natural ventilation │ │ │ │ │ │ │ │ and hot 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.