The best system of all is of course to have a manhole built of brickwork, with grooved channels at the bottom entering a syphon. In this way it is impossible for gas from the sewer to flow along the house drain into the house, and obstructions in the syphon, should they occur, are readily removed. This system was to a great extent introduced by Buchan, of Glasgow. Buchan's disconnecting chamber and trap is represented in Fig. 34.

(133) Grease Traps.

In sculleries of the middle class and the larger houses generally, quantities of hot liquefied fat are thrown into the drains; the fat directly it is cooled by the cold water inside the drain sets, and entangling various debris, is a potent factor in stopping a drain; hence it has been found necessary to deposit this fat in what is called a grease box or grease trap. A grease box is simply a rectangular small catchpit, connected on the one side with the scullery sink and continued on the other into the drain. Fig. 35 is a good form of grease trap; the fat floats so that by bending the exit pipe downwards, so long as the water retains its level, the escape of the grease into the drain is prevented. The grating is movable and can be taken off for the purpose of removing the accumulation of fat. Grease traps are necessary evils, but they are often productive, through inattention, of nuisance.

FIG. 35.

(134) Inspection of Drainage.

If a drain of a house enter a sewer large enough for a man to creep up, a small portion of the drain can be inspected from the sewer, by the aid of a bull's-eye lantern; at all events, it can in this way be readily ascertained whether the drain is brick or pipe, and whether it is damaging the sewer, and also whether it has a flap-trap or not.

Proceeding inside the house, all visible parts must be examined in great detail-sinks, waste-pipes, soil-pipes, and rain-water pipes,

all being methodically inspected, and notes taken of their position, apparent course, and delivery; the size and material should always be noted. In bad escapes of sewer-gas, the gas may blow up a defective pipe so strong that the hand alone will feel the current, or, if the current is not evident, the flame of a candle held above it will give signs of up or down draught. It is not enough to examine drainage in a quiescent state, it must also be examined at the time closets or waste-pipes are discharging; an escape of gas may often be made visible by sharp pulling up the closet-valve, and also by pouring a large volume of hot water down the drain. There are besides special tests-these are what are known as "the peppermint test," "the smoke test," and "the water test."

(135) Special Drain Tests.

The peppermint test requires two persons, one of whom takes a bottle of peppermint oil and pours about two ounces at the highest or other suitable point, and then follows it up with several buckets of boiling water, taking special precautions by shutting doors and other openings that the odour is not allowed to escape into the house, save through defects in the drain itself. The operator must of course, remain at the spot where he has applied the test, otherwise the odour remaining in his clothes will obscure all indications. His companion now examines all along the course of the drain, and if there is any smell of peppermint, it is a proof of defect. The addition of some strong ammonia to the peppermint oil is said to render the test more certain. As the principle of this test is the volatility of a strong-smelling substance, it is obvious that any volatile substance can be substituted for oil of peppermintfor instance, bisulphide of carbon, powdered iron sulphide followed by a strong acid so as to disengage sulphuretted hydrogen, light petroleum, ether, and other substances.

The smoke test is best applied at the lowest practical portion of a drain, if a special opening cannot be conveniently made, then a trap can be taken out and the sewer end of the drain blocked up by a plug. "Smoke rockets" are sold for the purpose of applying the smoke test; the rocket is lit, pushed into the drain, and the opening well covered with sacks or anything else which will keep the smoke in, a dense volume of smoke fills the drain, and bursts

through defective joints and places. There are also some very good machines in the market by which large volumes of smoke can be pumped into the drains. The water or hydraulic test is more applicable to new drains not covered in than to covered or old drains. A plug1 is put in the lowest portion of the drain, and it is then filled with water; if the joints are not cemented an escape takes place, and the level of the water sinks to a corresponding degree.

1 Plugs are in commerce; one of the best is an iron disc, with a deep flange in which is a thick ring of rubber. They are made of different sizes.

CHAPTER XVI.

SEWERS.

(136) Drain-Sewers.

WHERE the separate system is in use, as in Ely, Gloucester, Dover, and many other towns-that is where there are two sets of pipes and channels, the one for the sewage, the other for the storm and subsoil water, then the main sewers should be absolutely water-tight; and preferably constructed of socket-pipes. On the other hand, where there is no separate system, it seems best to take in the subsoil water, and therefore the mains should be constructed of brick. These latter kind of sewers Dr. Corfield calls "drain-sewers," because they fulfil a double function-they act like ordinary agricultural drain-pipes and drain away the subsoil water, and also as channels for the sewage. The chief points to be noticed in drain-sewers are the following:

(1) They must be placed at sufficient depth to drain the cellars.

(2) They should have a fall of from 1 in 20 to 1 in 250; the latter is not always possible; the least fall by which it is known that a good sewer can be kept free from deposit is 1 in 1,000.

(3) They should be egg-shaped in section, for it is capable of proof, that an egg-shaped sewer is cheaper to construct, is stronger, gives a greater "hydraulic depth," and therefore an increased velocity, than any other form.

(4) The flow in any sewer should not be under 2 feet per second nor over 4 feet per second; the former will just keep a properly constructed sewer free from deposit; the latter will unnecessarily increase the maintenance expense.

Sewers should be laid in as straight lines as possible, and placed in a bed which will not be likely to sink. Concrete as a bed is often used. The gradients must be true throughout. If curves are necessary, the radius of the curve should not be less than ten times the cross-sectional diameter of the sewer.

No sewers or drains should join at right angles or directly opposite the entrance of others: two sewers joining together should always do so in the direction of the flow of the sewage; at such junction it is often advisable to form what is called a bell mouth with a ventilating shaft.

Provision must be made for easy access to sewers for flushing and clearing them, without breaking up the roadway; this is effected by side entrances, manholes, flushing chambers, and the like. The position of these manholes is of great importance, they should certainly be never placed opposite to the entrance of a public school; it is from time to time necessary for the manhole door to be opened during several hours, and during this time sewer gas freely streams through the opening. In the writer's district there have been several just complaints from this cause; these complaints have been dealt with in some instances by removing the manhole or side entrance to a more suitable spot, in others by making stringent regulations that the manhole is only to be used in certain hours.

(137) The Flushing of Sewers.

Sewers should be flushed periodically, whether there is any deposit or not. The simplest flushing apparatus is any kind of dam which ponds up the water, and is capable of being suddenly removed. The power of water used in this way is surprising. In an experiment with a flushing gate, 4 feet high, the quantity of water ponded back up for one flush was 26,665 feet; three flushes carried brickbats 1,300 feet; the reason for this flushing power is not alone the simple mechanical force, but also the loss of weight of solids by immersion in water, a brick bat weighing several pounds in air weighs much less in water, and hence the mechanical force added to the buoyancy force of the water moves it. Formerly the deposits in the sewers were removed by hand labour, now such deposits are only removed by hand labour when flushing fails. Hand labour is expensive; it was stated in the evidence before the