The dangers to health from a properly-constructed system of house {191} sewerage, such as is now generally agreed upon by sanitary engineers, are so very small as to practically amount to nothing, being, in fact, less than those of a well-kept yard privy of a country house, setting aside altogether the question of water pollution. The real difficulties in the way are the expense of such a system, which is considerable, and the finding of skilled and honest workmen to construct it and keep it in repair. Not every one who chooses to style himself a sanitary engineer or a sanitary plumber is to be regarded as such, by any means, but the physician should make it his business to know who are really reliable in this respect, for he will constantly be called in for advice on this point by those who have learned that good plumbing is the only true economy, but who do not feel themselves competent to distinguish between good and bad work. The main points of a satisfactory system are the following.[3] [Footnote 3: For further details consult the following: _American Sanitary Engineering_, by E. S. Philbrick, N.Y., 1881; _House-Drainage and Water-Service_, by James C. Bayles, N.Y., 1878; "House-Drainage and Sanitary Plumbing," by W. P. Gerhard, in _Fourth Annual Report State Board of Health Rhode Island_, 1882; _The Sanitary Engineer_, a weekly journal published at 140 William St., New York City.] 1. All soil- and waste-pipes should be extended up to and through the roof, and be freely open at the top. The extension of the soil-pipe should be full size--_i.e._ from four to six inches in diameter. 2. There should be a fresh-air inlet in the house sewer just outside the house, and between this inlet and the main sewer should be a trap so arranged as to permit of inspection. This prevents the ventilation of sewers through the soil-pipes. If a perfect system of sewers, uniformity of house-connections, and uniform height of houses could be guaranteed, this inlet and trap would not be so necessary, although even then it would be useful. 3. Every water-closet, wash-bowl, bath-tub, sink, etc. should have a trap placed as close to it as possible. This trap is desirable, whether the discharge be into the sewer system or not. For example, a kitchen sink, the pipe from which passes to the outer air and discharges there, should be trapped, for this pipe is foul, and if it be untrapped will act as an air-inlet. 4. The nearer to the soil-pipe that the fixtures can be arranged the better. It is especially desirable to avoid the necessity for long horizontal waste-pipes from stationary waste-bowls and from bath-tubs. 5. Bell traps, D traps, bottle traps, and mechanical traps are objectionable. The S trap is, upon the whole, the best, but it should be provided with a vent-pipe to prevent siphonage.