contain a sufficient supply of the purer day air for the persons using them at night, and of mechanical means of taking down pure air from above the house to be a supply during the night. At a certain height above the surface of the earth, the atmosphere being nearly of equal purity all the earth over, a man rising in a balloon, or obtaining air for his house from a certain elevation, might be considered to have changed his country, any peculiarity of the atmosphere below, owing to the great dilution effected before it reached the height, becoming absolutely insensible. 'Now, in regard to the dilution of aërial poisons in houses by ventilation, I have to explain that 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 top 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 therefore, even in a calm day, what is called a chimney-draught proportioned to the difference. In England, therefore, of old, when the chimney breast was always made higher than the heads of persons sitting or sleeping in rooms, a room with an open chimney was tolerably well ventilated in the lower part, where the inmates breathed. The modern fashion, however, of very low grates and low chimney openings, has changed the case completely; for such openings can draw air only from the bottom of the rooms, where generally the coolest, the last entered, and therefore the purest air, is found; while the hotter air of the breath, of lights, of warm food, and often of subterranean drains, &c., rises and stagnates near the ceilings, and gradually corrupts there. Such heated, impure air, no more tends downwards again to escape or dive under the chimneypiece, than oil in an inverted bottle, immersed in water, will dive down through the water to escape by the bottle's mouth; and such a bottle, or other vessel containing oil, and so placed in water with its open mouth downwards, even if left in a running stream, would retain the oil for any length of time. If, however, an opening be made into a chimney flue through the wall near the ceiling of the room, then will all the hot impure air of the room as certainly pass away by that opening as oil from the inverted bottle would instantly all escape upwards through a small opening made near the elevated bottom of the bottle. A top window-sash, lowered a little, instead of serving, as many people believe it does, like such an opening into the chimney flue, becomes generally, in obedience to the chimney draught, merely an inlet of cold air, which first falls as a cascade to the floor, and then glides towards the chimney, and gradually passes away by this, leaving the hotter impure air of the room nearly untouched.