External ventilators, connected by flues to the inside, where valves, flaps, and shutters should be utilised to shut off and regulate the supply at pleasure. A common place to fix these is a little above skirting level, especially in window backs and recesses; and they should be supplied with sliding shutters, of hit-and-miss form, so that, by a slight movement of the shutter, the ingress of cold air may be regulated, or shut off when not wanted. This position for ventilators may be said to obviate the evils at its origin, inasmuch as it clears the air from the bottom of the room upwards; though it is apt to. cause undercurrents and unpleasant draughts, to remedy which ventilators are used of the Tobin class, which are fixed generally half or part of the way up the external walls. They consist of external open gratings, connected by flues either to wooden shafts - such as pilasters, etc., more or less ornamentally treated, which are often placed within the rooms - or at other times, where the shafts would be inconvenient and out of place, the inlet is made by an ornamental bracket, projecting into the room, and this is connected by a shaft or flue, within the walls, to the external air grating. The fresh air enters the room at the top of the bracket (fitted with a valve to open and shut), and the force of the air is expended upwards, so that no draught is caused.
Ventilators are also fixed at or a little below ceiling level, and they are constructed of an external open-air grating, connected by a short flue to the inside ventilator, which is constructed on the hopper principle, with closed sides, so that the current of air has a tendency to go upwards. These precautions, however, do not successfully combat down draught, inasmuch as, while they remedy one defect, they cause the air to come in with considerable velocity on to the heads of persons below, which is a great defect. Hence, of the three places in which air can be brought into a room, the best place is midway, as the above will explain.
Fresh air can also be supplied to a room by means of the window, and this, too, without causing draught. The stop-bead on the sill is made 2 or 3 inches deeper, so that, by raising the bottom sash, the air is admitted through the space made at the meeting rails of the sashes thus opened.