People who are afraid of foul air coming out of these inlets put on a valve with mica flaps, so that the air can blow in, but foul air cannot go out. But, if there are no D traps under the water-closets and sinks, if the pipes are straight and sufficiently large ventilators are used, if the ventilating pipes go up above the roof and are not protected from the action of the wind, you will never find foul air coming out at the air inlet though you will find that fresh air is drawn in. There can be no accumulation of foul air, and the air that may be occasionally forced out is the last fresh air that has entered. Should you, however, find foul air coming out you will know that there is something wrong with the drain, that the drain or siphon is plugged, so that this air inlet becomes most valuable in pointing out when anything is going wrong. Brick drains, says Eassie, are variously shaped. The worst sections are those upon which two upright sides of brick have been built upon flat stones, so as to form a bottom, and then covered over with other flat stones, because the bricks can never joint tightly with the stone, and there is always a leakage going on into the surrounding subsoil. One great objection to brick drains is due to the fact that they cannot be constructed sufficiently small to meet the requirements of a house, and consequently are seldom found less than 9 inches in diameter, which is far too large a sectional area to properly drain a house. However compactly and well-burnt the clay has been made into bricks, a brick drain has only a certain life, so to speak, before its decadence begins with the usual attendant danger. Its lifetime is longer or shorter according to the subsoil in which it is placed, the material used as mortar, the gradient at which it is laid, the sewage which it removes, and the quantity of water, and especially of heated water, which passes through it, but the consensus of opinion in their disfavour for use in the interior of a house is overwhelming, and a universal preference is accorded to drains formed of earthenware pipes. A second objection to brick drains, however well they may have been built, is their want of smoothness, especially at the bottom, whereby the effete matters are not carried easily away; and this want of smoothness is aggravated by the roughness due to the unequal perishing of the bricks.