Another means of ventilating apartments, and promoting a circulation of air, is by the use of fans propelled by mechanical means, which drive the collected fresh air through flues into the rooms, the inlets to the rooms, of course, being fixed at such a distance above the floor that all feeling of draught is obviated. The cold air is taken or drawn down a shaft into the basement, and passes through a fan, which is connected to a series of large flues, and these, in their turn, are extended into various independent smaller flues, which run- up the chimney-stacks or upcast shafts, and have inlets opening into the numerous rooms on the different floors above.

At other times the foul air is drawn out by mechanical means, at about the skirting level, into flues connected to shafts, having exits in roofs, towers, etc.; and this egress creates such a vacuum that fresh air is drawn in through other flues, having inlets into the rooms, 7 or 8 feet above the floor, connected to air inlet shafts collecting the fresh air from the open.

The supply of fresh air to rooms is a small matter in comparison with the warming and cooling of it in summer and winter, and this is a matter which is only in its infancy as yet; the only method in vogue at the present time consists purely of passing the air over or through coils of pipes, etc., containing hot or cold air, or over boxes of ice and steam-heated pipes; and this means is only applicable (1) to the "fan" process, which explains itself, and (2) in cases where coils can be fixed between the inlet ventilators and the space to be ventilated, so that all fresh air must pass over or be acted upon in a more or less degree by the hot or cold appliances. Where fans are used, the fresh air can also be washed on a screen - which is very desirable.

In arranging systems of ventilation great care should be taken that the water closet and sanitary apartments are ventilated independently of kitchens, dwelling-rooms, etc.

The egress of foul air can also be provided for by the fresh air driving in through top ventilators, or by extraction. The former method allows of various forms, including: -

1. Independent ventilating flues, constructed in the chimney-stacks, and carried up to the top and out into the open. These are the most •commonly used remedies, and their only disadvantage is their liability to down draught; but this can be obviated by the use of the mica flap ventilators previously mentioned, which will only act one way as extractors, a contrary action closing them, as in the case of a down draught.

2. Outlet ventilators may also be constructed in ceilings, a flue of zinc connecting the outlet to a flue into the stack or outlet vent in the roof, as just mentioned; and these have the advantage that they can be ornamentally treated, in that we can use perforated centre flowers and cornices as outlets - the former being very efficacious, when the warmth of the gas will help to draw the foul air to the outlet and drive it away.

Warmth in outlet flues will always increase the velocity of the outcast, whether burning jets be inserted at the bottom of the flues or the flues themselves be warmed or surrounded by warm smoke flues, care being taken that smoke flues are not utilised as ventilators, unless great care can be exercised and very superior and tight flaps be used.

The extraction of foul air is usually attained by means of patent appliances, called air pumps, which are generally self-acting. These patents mostly take the form of cowls, which are fixed on the top of the special shafts or on roofs, the various small pipes being brought together in a main shaft, at the top of which the extractor is fixed, and this draws out all foul air, or rather it increases the velocity, and causes an "upcast"; but the efficacy depends on the movement of the external air very much.

It should be noted that the various small flues must not be let loose into the main stack, inasmuch as the different currents would clash, and form an injurious whirl, as it were, which would cause an obstruction. They must be kept independent of each other, or be gathered together one by one as they get near the extractor.