the cost of maintenance of the sewers. A brick drain in connection with a brick sewer, if at all defective at the junction between sewer and drain, soon damages the sewer, by allowing the house sewage to trickle between the sewer wall and the earth and thus (129) General method of House Drainage.

The simplest case is that of a country house, provided with water-closets and having facilities for disposing the sewage upon the land.

In such a case the drain may be laid at the proper gradient mentioned above, and terminate so as to deliver into a Field's flush tank; it will in this case be unnecessary to ventilate specially the drain, but there should be syphon traps to all sink wastes, and the end of the drain should be provided with a flap-trap.

Field's flush tank is a tank with a syphon arrangement; when the liquid rises above the syphon bend the syphon comes into play, and the liquid is discharged with a rush; the reason why it is advisable even in small country houses to have an automatic flush tank to deliver the sewage upon land, is because it is found that in most cases in which the sewage is carried direct on to the channels on the land without the intervention of a tank a more or less foul deposit is formed at the outlet of the pipe.

Where dry earth closets are in use in country places, there is only the slop water to be got rid of, and it is sometimes recommended to discharge this direct into agricultural pipes laid at a suitable depth in the garden or other ground, the slop water thus escapes at various points and fertilizes the crops, but this soil pollution is only admissible in cases where it will not pollute drinking water, and even here it is preferable to use a tank, and discharge the sewage intermittently and with a rush, because it is found that the earth about the first few agricultural pipes, gets far more than its due share of sewage and there is liability to deposit.

A flap-trap should be placed on the sewer end of a drain, to prevent flow of sewage up the drain pipes when the sewer is surcharged.

No house drain should be a continuous channel for gas to flow from the sewer into the house, a syphon or disconnecting chamber being always interposed.

If a drain has to be diminished in size, it is best to use the tapering pipes made for the purpose.

The system most in vogue now in the better class of houses, is to disconnect the drain from the sewer by means of a disconnecting chamber and syphon. The disconnecting chamber is a sort of man-hole, the sides of brick, and on the floor are open earthenware channels in connection with the house drains, while on the sewer side of the chamber is a syphon; in this way, whatever odours or evil properties there may be in the house sewage, no gas from the sewer system is allowed to enter. The grating over the manhole makes a good air inlet, and if the drains of the house are also ventilated, the air should more or less continuously sweep through the system. The ventilation of the house drain should be by a pipe 4 inches in diameter, carried from the highest point of the drain above the eaves of the house, away from bedroom windows.

(130) Traps.

Traps are contrivances to confine sewer gas; they are either mechanical or "water seals." The simplest form of trap is what is called a flap-trap; as applied to the end of a drain the flap-trap is simply a small door, hanging from its hinge, it allows fluids to pass freely one way, but prevents more or less perfectly the passage of fluids or gases the reverse way. Provided the flap accurately fits and the hinge is in good working order, a flap-trap at the end of a drain prevents sewer gas coming into the house with any force. But as the junction of the flap cannot be made absolutely perfect, it does not prevent some small leakage of gas by diffusion; on the other hand such a trap affords a good barrier against rats, and also against the ponding back of the sewage from the sewer if the sewer should be running bore full. Other defects of the valve-trap are oxidation of the various parts, especially the hinges; the fact that when water is rushing through the drain the trap is necessarily open, and sewer gas can then pass freely.

In the Metropolis there is no systematic inspection of the traps to the mouths of the house drains; they belong to the house owner, but he has no means of personally inspecting them nor of keeping them in order. Without a doubt they should in all cases belong to the local authority, and form part of the sewer system.

Another form of mechanical trap is the "Bower's trap." The pipe to be trapped dips into a vessel containing a ball of indiarubber, which by its buoyancy seals the bottom of the pipe that

dips into the vessel of water which forms the water trap. There can be no doubt of the efficiency of the principle of this kind of trap, a familiar illustration of which is exemplified in the modern form of glass marbles closing the necks of aërated water bottles. The marble presses against a ring of india-rubber, and the greater the pressure of gas the firmer the seal. It is also exemplified in some of the best forms of mercurial pumps; for example, in the mercury pump described and figured in the author's work on foods. A glass float rises on the surface of the mercury, its conical end plugs the tube, and thus there is a very effectual valve.

(131) The Water Seal.

The commonest form of trap is, however, the water seal, which is an application of the water lute of the chemist.

The water seal may be destroyed in different ways:-(a) it may be forced, as for instance when there are two traps on the same line of drain and no opening between them and hot water is poured down the drain, the air will suddenly expand, and one or both traps will be forced, the air bubbling through; (b) by the water in the trap evaporating, this very often happens with the bell-trap, the shallow water ring becomes dry, and then gas escapes; (c) "syphoning," this may happen when from any cause the pipe running full bore creates a vacuum behind the column of water, and sucks the seal out of the trap. A good example is given by Baldwin Latham in his work on sanitary engineering (see Fig. 32). S is the soil pipe open at its head; D a pipe running from the wash-hand basin B; U is a urinal. Every time the basin was used it unsealed the urinal trap, and water put down the urinal untrapped B. This mutual untrapping was cured by the insertion of the ventilating pipe. The column of water acts in these cases as if it were a solid piston; as it passes a branch it sucks the water from the syphon; the general cure for this is, as in the case cited, the ventilation of the branches; (d) the seal of a water trap may also be broken by the capillary action of a thread or a piece of cloth having by accident got into the syphon and hanging over the lip of the trap; the water may in this way be slowly extracted, dripping away from the end of the cloth or thread and the trap left dry. If the water in a

1 Foods, their Composition and Analysis.

trap becomes saturated with foul gas, it may deliver up the gas from the house side of the trap; but this only occasionally happens.

FIG. 32.

Water seal traps of approved form, and those in which the abovecited conditions of unsealing are not present, may be considered as perfect barriers to sewer gas.

(132) Examples of Traps.

One of the most common, and at the same time the worst traps, is what is known as the "bell trap" (see Fig. 33). It consists of

T

two parts, a fixed iron box with a pipe connected with the drain, and a movable bell-shaped cover; the rim of the bell dips into the water around the vertical pipe. The following are the objections to the bell trap: the cover is so easily removed that as a fact it is more often off than on; when once removed sewer gas has free channel, the thin film of water forming the seal affords no adequate

FIG. 33.

resistance to sewer gas; slight pressure forces the seal; the circular channel for the water is a natural receptacle for dirt and dust; it rapidly gets filled up, and then of course the trap is useless; lastly, from its construction, in dry weather the water rapidly evaporates, thus abolishing the seal.

An old form of trap, still used in the country, is the " dip-stone trap." It may often be seen in brick drains, placed so as to interpose a barrier between the house drain and the sewer. The drain is deepened at the spot, a piece of slate or stone placed tight across the drain but leaving a space of about 3 inches beneath. Water remains in the deepened part, forming a fairly good seal. It is also sometimes interposed in the course of a pipe drain, and in that case forms a bricked rectangular cavity. The objection to this form of trap is the liability to accumulation, and such traps are often little better than small cesspools. Dr. Corfield considers that these forms of traps may be much improved by making the ends nearest the house nearly vertical, giving the opposite one a gentle slope, and fixing the dip-stone so as to slant in the direction of the stream, and rounding off the inside with concrete, rendered in cement so that there are no angles or corners.

Stoneware syphon traps are superseding the forms mentioned. It is now a common practice to insert a stoneware gully trap between the house and the sewer, as

well as to have all the yard and surface traps stoneware syphons. Experience has, however, amply proved that a syphon connected with a sewer through which all the house drainage passes gets very frequently choked up; it is therefore necessary to provide some facility for opening or cleansing the syphon. For this purpose an upright piece is made from the lowest piece of the syphon, which may be continued up to the surface of the ground. It is better still to make an opening for inspection at the house

FIG. 34.

end of the syphon, so that instead of being closed with a tight lid, a grating may be placed over it and it may be used as an air inlet.