Human beings, on an average, require, per head, 300 cubic feet of fresh air per hour to keep them in a healthy state; and for this purpose they must be supplied with that amount of cubic space of pure fresh air, refilled and circulated by the natural pressure of the ingress of the wind, or artificially by fans, from behind, as it were, to drive out the foul air, and replace it by pure air; or that object is attained by extracting the foul air from the top and drawing in the pure air from below.

Assuming that each person requires 300 cubic feet of air per hour, the air in a room 10 feet x 10 feet and 10 feet high, would require to be changed three times in ten hours for one person; so that rooms of various cubic contents require the air they contain to be changed according to the amount of air available for each occupant.

There is no law laying down the superficial area or cubic contents which each private dwelling should have per inhabitant; as such restriction or regulation would be unreasonable and impracticable, though it is generally laid down by local authorities that rooms of less area than 100 feet superficial, by 9 feet high, should have special ventilation beyond that which an unused fireplace with window and door would give. Nevertheless the Local Government Board require that, though 300 cubic feet may suffice for each individual in public dormitories, yet in practice such cubic space should never be less than 850 for sick persons; and, indeed, some hospitals have as much as from 1,000 to 1,200 cubic feet, and more, per patient. The Educational Department require 130 cubic feet per scholar, while lodging-houses should have 30 feet superficial, or (assuming 8 feet high) about 240 feet cube per person. These Fig.ures will give the student an idea of what is required, and also show him how much really depends on the circulation of fresh air.

## Draughts

The various methods for promoting circulation hereinbefore mentioned, having their inlets and outlets by means of flues or ventilators, are apt to cause draughts of various kinds, which it is necessary should be obviated as much as possible; and, with that object in view, a separate supply of air should be provided to feed the fires, which require, under ordinary circumstances, 150 feet cube of air per minute. Unless that supply is made from a special source, it will be drawn to the fire from the door or window, causing a draught between the inlet and outlet; and, consequently, a fire should have an inlet of air contiguous to it, either at the back or sides of the fireplace opening, having an area nearly equal to the outlet, flue, or opening at the chimney-pot.

Down draughts are equally as troublesome, though not so regular, as the fire causes an up draught. Moreover they are not perceptible, unless we have artificial means of ventilation, such as flues and ventilators, which at times are converted, by circumstances, temporarily into sources of this defect; and this all good patents and remedies should render impossible.

Draughts sometimes are driven down outlet flues; and this can be remedied, or reduced to a minimum, by the use of flap ventilators made of mica or other light material, which can be moved or opened by an up or outlet draught, and closed by a down draught. At other times they are caused by the air rushing through the inlet, which strikes against anything such as a ceiling, and comes down on people's heads; or the stagnation of the foul air at the top will press it down.

The heat of a fire or hot-water pipes will draw from windows and skylights in very cold weather, and create serious draughts; hence in the best hot-water work provision is made for running the pipes around the skylights or intermediately, so that the cold air has to pass the heated pipes.

Common sense gives us the remedies for these: either by increasing the outlet for foul air, or by regulating the supply; though this latter is not to be recommended.

It should be understood that a larger inlet than outlet increases the velocity with which the air circulates, and drives out the foul air all the sooner.

Ingress of pure air can be, or rather should be, of two kinds - warm for winter, and cold for summer. It can be brought in by the following methods, among others: -