2178. Ventilators, &c. The general mode of renewing the air, is by opening the sashes or doors of the house, in periods when the exterior temperature and weather are such as not to injure the plants within. The cool air of the atmosphere, being then more dense than that of the house, rushes in till it cools down the air of the house nearly to an equilibrium with that without. The next mode most common is that of having a range of boards hinged to oblong openings, in the lower and upper parts of the house, and generally in the front and back wall; those in the back wall opening to the south, or having the opening otherwise guarded, so as to prevent the rushing in of cold north winds. Sometimes these ventilators are made with a cylinder and fans to extract the air, and sometimes, as most generally, they are mere openings of small dimensions; but, in order to effect any circulation or renewal with this sort of ventilators, the opening must have an area of 2 or 3 feet, and there must be a considerable difference of temperature between the air of the house and the open air.