ous when applied to actual use, but the profits on their sales are large, and the introduction of better and more simple forms of apparatus would seriously disarrange the existing order of things. The abolition of the trap-vent laws, for example, would cut out a thousand pounds or more of useless iron and lead pipes from the plumbing of every good-sized house. Manufacturers and dealers would suffer heavy losses, and so the vast interests involved are carefully guarded by all the resources which money can command in municipal legislation. This influence has extended even to the press; and we see tradesmen's journals persistently ignoring scientific progress and upholding still the old methods which are to enrich unscrupulous manufacturers and their clientage of uneducated mechanics. It is for the interest of invested capital that plumbers be kept as ignorant and as unprogressive as possible.
In this conflict of ignorance and prejudice with science, it is not difficult to trace still further the cause for so much popular distrust. If the most common defects in the apparatus for ordinary house-drainage could be clearly understood, it would be readily seen that the want of confidence in plumbing appliances arises mainly from a general misapprehension regarding their real imperfections. We know, for example, that sewer-air, or sewer-gas, as it is improperly called, finds its way continually into many houses, and frequently causes disease and death. How does this sewer-air gain an entrance? If you consult your plumber, he will deny that there is any possibility of such a defect existing in the drain-pipes and fixtures he has put in. But your physician will tell you that the symptoms of illness of some member of the household show unmistakable evidence that the patient has been poisoned by sewer-air. A thorough examination of the drainpipes shows that they are securely jointed, and that there are no leaks in them. Nevertheless it is certain that sewer-air gains an entrance to the house in considerable quantities, and after a time it is discovered that the poisonous air finds its way in through the traps attached to the basins and sinks and water-closets. A word of explanation may be necessary in regard to these traps. Although of a great variety of forms, they are all essentially a device for allowing waste-water to flow through them from the fixtures to which they are attached into the drains, and to prevent air or gases from passing in the opposite direction into the house. The resisting medium in most cases is a water-seal, consisting of a small body of standing water in the body of the trap. When it is found that sewer-air passes freely through these very traps that have been designed to keep it out, the inference is almost irresistible that the water-seals are at fault, and that water is not a suitable medium to be used for this purpose, since air from the drains apparently forces an entrance through it.
