ARNOTT'S VENTILATOR. Dr. Arnott's contrivances in respect to heat and air all de pend on these two points—how to bring pure air into a room and a fire ; and how to expel impure air from a room and a fire. The Ven tilator, now happily so much used in ordinary rooms, is an exceedingly simple means of effecting the second object. When the cholera was raging in London in 1849, Dr. Arnett suggested that, in every badly ventilated room, a brick should be taken out of the wall near the ceiling, so as to open a direct communica tion between the room and the chimney ; and he wrote a letter 'to the Times, in which the rationale of this contrivance was described with a clearness and simplicity which few but himself can effect by words alone. He said, Every chimney in a house is what is called a sucking or drawing air-pump, of a certain force, and can easily be rendered a valuable ventilating pump. A chimney is a pump— first, by reason of the suction or approach to a vacuum made at the open end of any tube across which the wind blows directly ; and secondly, because the flue is usually occupied, even when there is no fire, by air somewhat warmer than the external air, and has, there fore, even in a calm day, what is called a chimney- draught proportioned to the dif ference. In England, therefore, of old, when the chimney breast was made higher than the heads of persons sitting or sleeping in rooms, a room with an (glen chimney was tolerably well ventilated in the lower part, where the inmates breathe.' But modern fire-places are made low, and the heated air cannot thus escape. If, however, an opening be made in a chimney-flue through the wall near the ceiling of the room, then will all the hot im pure air of the room certainly pass away.... For years past I have recommended the adop tion of such ventilating chimney openings ; and I devised a balanced valve, to prevent, during the use of the fires, the escape of smoke to the room.'
Such is the philosophy and the action of the ventilating valves. They are made rectan gular or circular, of iron or of brass, plain or ornamental, balanced in front or behind ; but all act alike on the same principle.
Dr. Arnett has devised other and more ela borate ventilators. He was applied to by the Board of Health, a year or two ago, to suggest means for the ventilation of ships ; and he has described very efficient means of attaining this end. Quite recently (ism) he has planned a system of ventilation for a large building of a highly curious and scientific kind. The building is the York County Hospital, which has a reservoir of water at a height of 00 feet from the ground ; and he resolved that this water should work the apparatus for ventila ting the whole building. A large circular vessel has been constructed, which is partly filled with water ; floating on the water, or rather resting with its edges on the water, is a kind of gasometer, or inverted air-chamber. A pipe of small diameter extends verti cally from the water vessel to the reservoir at the top of the house ; and the water, flowing down this pipe, ex!srts a great hydraulic pres sure on a piston, which forces the water into the gasometer, and forces the air from the gasometer into air.passages which ventilate the building. The scientific principles of the hydraulic press, of the air-pump, and of the gasometer, are all involved in the arrange ment. The gasometer holds 125 feet of air; it is filled and emptied by the action of the apparatus, sixteen times in a minute ; so that 2000 cubic feet of air per minute are forced into the ventilating passages of the hospital.
The general subject of ventilation, in its varied forms, is noticed in the article before referred to. [IVAalunm AND VENTILATINIG.]