This subject has long been left in a very unsatisfactory state of neglect, despite its importance with regard to health. The following remarks are mainly gathered from a paper on the subject recently read by Arthur Walmisley before the Civil and Mechanical Engineers' Society, in which he reviews the principal systems.

As regards window ventilators, Lockhead's perforating panes of glass are a useful form when placed in the highest pane of the window farthest from the fireplace. A system in very general use is found in Moore's patent ventilator, which consists of glass louvres fixed so as to open at any angle required with facility by means of a cord, which, when set free, allows the louvre plates to close of themselves airtight. Moore's sliding glass ventilators, which are usually made in circular plates of 9 in. or 10 in. diameter, with egg-shaped openings neatly cut and turning on slips of glass with bevelled edges, are very effective for the admission or extraction of air in a room, but admit the rain in wet weather. Another method of admitting fresh air to a room consists in leaving an aperture in the external wall, at a level between the ceiling of one apartment and the floor of the room immediately above, then to convey the fresh air through a channel from the external wall to the centre of the ceiling of the apartment below, where the air can be admitted by an opening and dispersed by having a flat board or disc to impinge against, suspended 4 in. or 6 in. below the opening of the ceiling, and so scattered over the room.

The cold air, however, thus admitted, plunges on the heads of the occupants of the room and mixes with the hot air which has risen near the ceiling. A top window-sash lowered a little to admit fresh air has the same disagreeable effect, the cold air being drawn towards the floor by the chimney draught, and leaving the hot air to stagnate near the ceiling. In any siphon system placed vertically the current of air will enter by the short arm, and take its exit by the long arm, and thus the chimney flue acts as the long arm of a siphon, drawing the fresh air from the nearest opening. Fresh air may be introduced through perforations made in the woodwork of the bottom rail of the door to the room, or through apertures in the outer wall, admitting the fresh air to spaces behind the skirting board, and making the latter perforated. The only objection to this plan is the liability for vermin to lodge between the skirting board and the wall. This may be prevented by covering the outside apertures with perforated zinc, but such covering also helps to keep out the full supply of fresh air.

Butler recommends, while admitting the cold air through side walls near the floor level, and allowing the foul air to escape at the ceiling, that the fire draught should be maintained quite independent of the air inlet to the room, the requisite amount of air for combustion being supplied by a separate pipe led through the hearthstone with its face towards the fire, the latter acting as a pump, which is sure to procure its own allowance from the nearest source; thus the draught which would otherwise be felt by the fire drawing its supply from the inlet across the room is considerably reduced. The foul air may enter the ceiling in the centre, and be conducted by an air-flue either to the outside or to the chimney. The chimney is the best extractor, as its heated condition greatly favours the ventilating power.

Dr. Arnott was one of the first to draw attention to the value of a chimney as a means of drawing off the foul air from the interior of an apartment. He invented a ventilator consisting of a well-balanced metallic valve, intended by its instantaneous action to close against down draught and so prevent the escape of smoke into a room during the use of fires. If the fire is not alight, what is known as the register of the stove should beclosed, or a tight-fitting board placed in front of the fireplace, with the adoption of all chimney-ventilators fixed near the ceiling.

A very ingenious device was described by Prof. Morse at a recent meeting of the "American Association for the Advancement of Science," held in Minneapolis, having for its object the utilization of the sun's rays for warming and ventilation. The device consists mainly of a slaty surface painted black, placed vertically on the out side wall of a building, with flues to conduct the warm air to the inside. The slates are inserted in a groove-like glass in a frame. A library measuring 20 ft. by 14 ft., by 10 ft. high, was warmed in this way by an apparatus measuring 8 ft. long by 3 ft. wide, and was thus kept comfortable throughout the winter except on a few of the coldest days. Prof. Morse states that as a general result of the experiments a difference of 30° could thus be secured during 4 or 5 hours of the day. He found in the morning that when the sun's rays rested directly on the apparatus the air passing through it was raised about 30°, and that it discharged 3206 cub. ft. of warm air per hour. The sun, by heating the solid objects upon which its rays fall, causes a gentle and regular circulation of air along the surface of the ground. This fact suggests the advantage of so placing a building that a maximum amount of sunshine is admitted into the rooms most occupied.

Where air without the sun's heat is required, as in the case of meat markets, the method adopted in the design for the Metropolitan Cattle Market may be recommended, where 5 louvre boards, each 8 in. by 3/4 in., are made to revolve on pins fixed near the lower ends of support; these louvres open or close by means of a chain passing over pulley blocks.

In America the plan most generally adopted for the ventilation of some of their large institutions is to admit the fresh air in the middle of the room, after warming it by a stove or other heating appliance, placed either in the room or in another compartment, and connected by an air-duct to the centre. The air so admitted first ascends to the ceiling, and then is supposed to be drawn down from apertures near the floor in the walls of the room, whence it is allowed to escape by passages to the smoke-flue, and so to the outside. In some of their hospitals fresh air is admitted through a series of long narrow apertures, covered with a perforated plate, situate one over each bed a little above the patient's head, and drawn out through a tube at the foot of the bed, which is placed in communication with a suction flue, the object of this arrangement being to free the neighbouring patients from the danger of inhaling the heavy gases generated in disease.