There is generally more or less draught in a chimney even without a fire, from the air within being slightly warmer than that without; and this action might be strength ened by burning a jet of gas within the ventilating aperture at v. Where a house is to be built new, some recommend having special ventilating-flues in the walls, separate from hut close to the fire-flues, so that the air may be heated, and an ascending current pro duced. In weather when fires are not required, the draught can be maintained by gas jets at the entrances to the vents. This plan of causing a draught by gas is applicable to churches and apartments without fire-places.
Where a fire is burned for the express purpose of producing a current of air, it is called ventilation by fire-draught. The plan has been exemplified with success in mines, where .a fire being lighted at the bottom of a shaft, air is drawn off in all direc tions around, and sent up the shaft; to replace which, fresh air is constantly pouring down other shafts.
Many of our large buildings are ventilated by fire-draught. A school or church may be ventilated by having the flooring perforated with holes, through which air, warmed by hot-water pipes, passes to the interior. The ceiling is perforated, leading to a chamber which communicates with a vertical flue which leads to the fireplace of the warming-ap paratus, situated at the foot of a flue. As the only air which reaches this must pass from the vertical flue, a constant current is maintained therein, and also through the apertures in the ceiling. Dr. Reid exemplified this method, first in his own class-room in Edin burgli,and afterward in various public buildings, among others, in the temporary house of commons, erected after the burning of the old house in 1834. The plan was attended with some inconveniences—in fact, no plan can meet every contingency—but, notwith standing the storm of hostile criticism that was raised at the time, prof. Tomlinson (Treatise on Warming and Ventilation, 1864) gives it as his opinion that " in the case of the temporary house of commons, where all the arrangements were left in his own hands, he succeeded in the proposed object of removing the vitiated air, and keeping up a constant supply of warm or of cool air to fill its place." The arrangements for warming and ventilating the present house of commons are a modification of Dr. Reid's plan In other cases, as at the prison in Milbank, warm air is admitted at the ceiling, and carried off by the draught of a chimney in connection with the sides or lower part of the rooms.
In these last-mentioned instances, the apparatus provides as Well for the admission as for the removal of air. In ordinary dwellings no special provision is in general made as to admission. It is, in fact, not absolutely necessary; for the removal of a portion of the air of a room never fails. to secure the entrance of a fresh supply somewhere. When
ever the chimney-draught or other means removes a little of the pressure inside the room, the pressure without forces air through every opening and chink; and even, were there no actual openings, would force it through the porous substance of the structure— such as mortar, and even wood itself. But this irregular source of supply has various inconveniences. It often requires more force to strain the air in this manner than the draught is possessed of, and then the chimney smokes; it is smoke produced by this cause that is curable by opening the door or window. Another objection is that impure air is often thus drawn into rooms from the lower parts of the building and from drains about the foundation. For these and other reasons there Ought, in all cases, to be a free and legitimate entrance provided for fresh air, so as to give a control over it; and this entrance should be independent of the windows. It is a much disputed point where about in a room the air should be made to enter—some advocating openings for it near the floor, others near the ceiling; and it must be confessed that neither method has yet been rendered unobjectionable. One essential thing is to prevent the air from rushing in•ith a strong current, by passing it through minute holes spread over a large space. A tube, for instance, leads from the outer air to a channel behind the skirting. or behind the cornice, and the air is allowed to issue into room throueli minute holes, or through a long, narrow, and concealed opening covered with perforated zinc or wire gauze. The passage or tithe leading from outside the wall can be more or less closed by a valve regulated from the inside.
But the great diffieultv lies in the coldness of the air directly introduced from the outside, whether by the doors and windows, or through channels in the walls, and all such plans of ventilation must be considered as imperfect makeshifts. The fresh air ought in every case to e warmed before being admitted, or, at least, before allowed to circulate in a sitting-room. In the smokeless grate (fig. 1) the air is led directly from the outer atmosphere into a channel underneath the hearth, and escap ing below the fender and about the fire, is warmed before spreading through the apartment.W With stoves and l.eated pipes, the air should enter about the heated sur face; in stoves on the cockle principle, the fresh air, as it enters, is made to pass between the casings of the stove. With an open fire a very feasible plan is to make the fresh-air channel pass behind the fire-place, and allow the warmed air to escape from concealed openings about the chimney-piece and jambs, or from behind the skirting. In Coudy's ventilating-grate,.the fire-box is constructed of hollow pieces of fire-brick communicating with the external atmosphere and with the room.