The following conditions are essential for its proper action. The curve must be of such a depth as to ensure a height of not less than 3⁄4 inch of water always standing above the highest level of the water in the curve. The
outlet pipe attached to the trap should not be too small nor have too sudden a fall as it leaves the trap, otherwise when ‘running full’ of water, all the water will be sucked out of it by the pipe beyond, owing to the too narrow bore and too perpendicular inclination of this latter.
The midfeather trap consists of a round or square box or receptacle, into the upper part of which, on one side, an inlet pipe discharges, whilst at a corresponding height on the opposite side there is an outlet pipe. The upper part of the box is divided by a partition, which dips at least 3⁄4 inch below the surface of the water, always standing in the receptacle, at the level of the outlet pipe. The principle, therefore, of the midfeather is similar to the syphon trap. The receptacle is so arranged that any heavy substances collected at the bottom can from time to time be removed. A useful variety of the midfeather is ‘Dean’s patent drain-trap,’ manufactured by Edwards, of Ruabon.
The ball-trap is not in very general use. By this arrangement the drain is trapped by means of a hollow ball, which rises with the water in the drain until it is carried against and closes an orifice.
The common ball-trap is stated to be inefficient and unsatisfactory. The facility with which it can be removed or placed out of gear often leads, in the hands of careless servants, to the untrapping of the drain altogether. A good description of common sink trap is Antel and Lock’s, shown in the accompanying engraving, which explains itself.
Amongst the circumstances that impair the efficiency of house-traps may be included the neglect to allow the passage of water through them sufficiently often, and with force enough to flush and cleanse the trap, and renew the water in it. The results are, that the water becomes saturated with sewage exhalations, which escape into and contaminate the air in the house; and that the trap becomes either dry or choked up.
Another contingency to which house-traps are exposed when the drains are made to form a continuous and disconnected system with the sewers, is that of the water being sucked out of the trap, owing to the combined effect of the pressure of sewer air and the aspirating power of the house, into which the sewer-gas would then pass unchecked.
In our articles Drainage and Sinks, we have pointed out the peril attaching to the
intimate connection between the house-drains and sewers, and given in the former practical directions for its avoidance.