Tightness of all waste-pipes being secured, the next point in order is their proper ventilation. A good deal has been said, and little has been proved, about the different effects on the human system of the gases of decomposition which have been produced in the absence of a sufficient circulation of air, and those produced where the ventilation and dilution are more complete. The probabilities of the case are, of course, entirely in favor of the latter condition, and it is accepted by all sanitarians as an axiom that all water-ways and all vessels in which organic decomposition, even the decomposition of adhering slime, takes place, should be ventilated as thoroughly as possible. Until about ten years ago nearly all waste-pipes were tightly closed at the top, and were shut from the sewer by a trap at the foot, allowing absolutely no communication between the outer air and the atmosphere of the pipe except as fresh air might be carried in through the water-seals of the traps at each end. At about that time it was becoming the general custom in the better class of work to carry a small vent-pipe, often only one inch in diameter, rarely more than two inches in diameter, through the roof of the house, closing it at the top and perforating it with a few inefficient holes. This had undoubtedly the effect of relieving the pressure on the atmosphere of the pipe caused by the filling of unventilated sewers with tide-water or storm-water, or by a sudden increase of temperature from the admission of hot water. Later, it was accepted as a universal rule, and it became a quite general practice, to carry the soil-pipe above the roof with its full diameter, providing its summit with some form of ventilating cowl. All this constituted not ventilation, but venting. Real ventilation was introduced only with the very recent improvement of admitting fresh air at the foot of the soil-pipe, so as to make a complete circulation from one end to the other--a circulation sufficient to produce, by the diffusion of gases, a very fair ventilation of lateral waste-pipes of moderate length. It is now coming to be understood that ventilating cowls, of whatever form, are an obstruction to the movement of air in the absence of wind, and that, as what is needed is never a vigorous current, but always a living one, these cowls had better be dispensed with. We have learned, too, that the most efficient means for increasing the flow of air through the top is to increase its diameter at the top, enlarging the highest length of a four-inch pipe, for example, to a diameter of six inches. With this arrangement, and with a foot-ventilation four inches in diameter opening at a point where it can never be obstructed by rubbish or by snow, there will be secured a condition perhaps more efficient in improving the condition of an imperfectly drained house than any other one thing that may be done.