It is exceedingly difficult to persuade people upon this point; but such is the case. When the rain-water pipes starting from the top of the house below the bedroom windows, and frequently behind parapets, so that any air that comes out at the top comes out exactly close to the bedroom windows, and when these pipes come down straight into the drains and so ventilate the drains, foul air from the drains gets into the house, and disease is the result. But it is more difficult to make people understand that even when these rain-water pipes are trapped at the foot they are almost as dangerous as the untrapped ones, because foul air from the drains will pass gradually through the water in the traps into the pipes, so that these pipes are always filled with foul air and contain gases that have come from the drains. As soon as it rains, water passes down, and the air of these pipes is displaced, comes out at the top, and so if these tops are near the windows of rooms, cases of disease will happen in those rooms. The rain-water pipes ought to discharge on to the surface of the areas, where there ought to be siphon gullies connected with the drains. _Ventilating Pipes._--While on the roof we can look around and observe the ventilating pipes: 1st, whether there are any or not; 2nd, of what size; 3rd, whether they have cowls or not; and 4th, in what positions they are. If we observe that they end at the top, near to chimneys, we shall see that there is liability, on account of the down draught, of the foul air from these ventilating pipes passing down the chimneys. Chimneys often have down draughts, and if ventilating pipes are placed near them, the foul air may pass down into the rooms. If, on the other hand, although not ending near the tops of the chimneys, they are placed close to the chimneys or to walls so that their tops are sheltered, they will not act properly, and they ought to _be carried above the ridge_ of the roof, and end away from walls or chimneys. The same rule applies to chimney tops, they should not be sheltered by higher buildings. _Cistern._--The first thing we come to inside or just below the roof (or perhaps on the roof), is the cistern. The first point to observe is the material of which it is made. Lead cisterns (and so, too, galvanised iron cisterns) are affected by certain kinds of water; and it is important, in certain places, that cisterns should be used which are not capable of being affected by the water. Galvanised iron cisterns cause certain forms of poisoning with some waters. However, as a matter of fact, both lead and galvanised iron cisterns are used enormously, without any serious results following from their use.