Besides the traps under basins, sinks, etc., there is usually a trap wherever a drain-pipe runs out of the house to the street sewer. This doubles the protection. These traps are sometimes outside the foundation wall, sometimes in the wall and occasionally inside the cellar. One should know where they are in case anything is the matter with them, and also in order that one may not put up a shelf for milk, or a bin for potatoes directly over the spot where a trap is. Drainage systems always have to be ventilated. A pipe which extends out of the roof of the house, or runs up the side beyond the eaves, or comes up from the foundation or the lawn with a hood over it, is a ventilator for the drainage. They help to make the air pressure right in the pipes and they prevent the gathering of foul explosive gases. As they are vents for such things, one does not want them close to a roof window, nor under a veranda, nor anywhere except in the open upper air. _Ashes and Garbage._--To say that only liquid substances should be poured down waste-pipes seems a needless repetition of what everybody knows, yet it is knowledge constantly disregarded and sometimes forgotten even by careful people. Waste substances not suitable for the pipes have to be somewhat classified. Cities and towns have different regulations for the disposal of waste and sometimes one is required to do a good deal of sorting in compliance. There are, however, three general classes of waste; ashes, garbage and trash. Nothing should be put into ashcans except ashes. Garbage is the waste from food, or any substances which are wet or subject to decomposition. Trash is papers, cans, bottles, egg shells, glass, hair, dust, broken objects of all sorts and kindred things. This class may have to be subdivided several times for the convenience of people who remove it, but the three main divisions in house waste are made not on account of requirement but for the sake of neatness and decency.