into the house when upper closets or sinks were used. 5th, Traps may perhaps be inefficient from the pressure of the sewer air, combined with the aspirating force of the house displacing the water, and allowing the air uninterrupted communication between the sewer and the house. The extent of the last danger cannot be precisely stated. From a long series of observations on the pressure of the air in the London sewers, Dr. Burdon-Sanderson ascertained that in the main sewers, at any rate, the pressure of the sewer air, though greater than that of the atmosphere, could never displace the water in a good trap. In a long house drain which got clogged, and in which much development of gaseous effluvia occurred, there might possibly be for a time a much greater pressure, but whether it would be enough to force the water back, with or without the house suction, has not been yet experimentally determined. Dr. Neill Carmichael has shown that water siphon traps act efficiently so long as they

are not emptied by any siphon action beyond. But the reasons already given show that we ought not to place dependence solely on traps,' though they are useful adjuncts. In arranging the house pipes, the sink and waste-water pipes must not be carried into the closet soil pipes, but must empty in the open air over a grating. See Fig. 85. In the case of soil or water-closet pipes there must be also a complete air-disconnection between the pipe and drain by means of one of the contrivances now used by engineers. At the point where this disconnection is made, there ought to be some easy means of getting at it for inspection.

3

A good simple form is Buchan's trap (Fig. 86). A good form of manhole is Mr. Rogers Field's (see Figs. 88 to 90). Professor Reynolds' has suggested an arrangement which seems fairly good and simple.

A simple trap is made by inserting a pipe in the centre of a siphon, and carrying this pipe to the surface, or higher if considered desirable. It is, however, apt to be clogged with grease, fæces, and other light matter rising into the pipe. There are various similar arrangements. The "Somerset Patent Trap," designed by Mr. Honeyman, and much used at Glasgow,

"Honestly speaking, traps are dangerous articles to deal with; they should be treated merely as auxiliaries to a good drainage system."-EASSIE.

2 For the sake of appearance in some cases, it may be necessary to carry the pipe immediately under the grating, but care must be taken that nothing occurs to obstruct the free communication with the open air through the grating.

3 From Mr. Field's By-Laws for Uppingham, with later improvements. I am indebted to Mr. Field for several valuable suggestions.-[F. de C.]

Sewer Gas, by Osborne Reynolds, M. A., Professor of Engineering at Owens College, Manchester, 2d edition, 1872.

is a midfeather-trap with an air-shaft on each side the partition; on one side the shaft ventilates the pipe leading to the

sewer, on the other allows fresh air to pass into This second shaft also allows

the house pipe.

the trap to be cleaned.

Rain-water pipes are sometimes used to ventilate drains, but independent of their small size, which often leads to blockage, they are often full of rain, and cannot act at the time when ventilation is most required. They are also apt to deliver sewer gas into garret windows. The plan is objectionable, and ought to be abandoned.

A good form of disconnecting trap for sink and slop waters is Dean's, which has a movable bucket for removing deposit (Fig. 91).

FIG. 91.-Dean's Gully Trap. A, Handle of movable bucket.

In yards, gully traps of different kinds are used, the action of which will be at once understood from the drawing (Fig. 87).

Examination of House Pipes and Traps.

Pipes and traps are generally so covered in that they cannot be inspected; but this is a bad arrangement. If possible, all cover and skirting boards concealing them should be removed, and the pipe and trap underground laid bare, and every joint and bend looked to. But supposing this cannot be done, and that we must examine as well as we can in the dark, so to speak, the following is the best course:-Let water run down the pipe, and see if there is any smell; if so, the pipe is full of foul air and

wants ventilation, or the trap is bad. If a lighted candle, or a bit of smouldering brown paper, is held over the entrance of the pipe or the grating over a trap, a reflux of air may be found with or without water being poured down. It should be noticed, also, whether the water runs away at once, or if there is any check. This is all that can be done inside the house; but though the pipe cannot be disturbed inside, it may be possible to open the earth outside, and to get down to and open a drain; in that case, pour water mixed with lime down the house pipe; if the whitened water is long in appearance, and then runs in a dribble merely, the drains want flushing; if it is much colored and mixed with dirt, it shows the pipes and trap are foul, or there is a sinking or depression in some part of the drain where the water is lodging. The pipe should then be flushed by pouring down a pailful of lime and water till the lime-water flows off nearly clear. The drain should also be blocked, and water poured into the house pipe to see if it be water-tight in every part.

Yard-traps are often very foul, and if the trap-water be stirred, gas bubbles out, which is a sign of great foulness, or that the traps are seldom used.

Main Sewers.

The outside house drain ends in a channel which is common to several drains, and which is of larger size. These larger sewers are made either of round glazed earthenware pipes from 15 to 24 inches diameter, or of well-burnt impervious brick moulded in proper curved shape and set in Portland cement, or stoneware bricks are partly used. The shape now almost universally given, except in the largest outfall part, is that of an egg with the FIG. 92.—Brooks's combined small end downward. Engineers take the greatest Drain and Subsoil Pipe. care with these brick sewers; they are most solidly put together in all parts, and are bedded on a firm unyielding bed. Much discussion has taken place as to their size, but the question is so complicated by the admission of rain water, that it is difficult to lay down any fixed rule, at least as regards the main pipes. All other sewers, however, should be small, and with such a fall as to be self-cleansing.

Sewers should be laid in as straight lines as possible, with a regular fall; tributary sewers should not enter at right angles, but obliquely; and if the sewer curves, the radius of the curve should not be less than 10 times the cross sectional diameter of the sewer. Sometimes there is an arrangement for subsoil drainage under a pipe drain, as in the plan proposed by Mr. Brooks.

The fall for street drains is usually from 1 in 244 to 1 in 784, according to the size of the drain. The flow through a sewer should in no case be less than 2 feet per second, and 3 is better. As in the house drain, the fall should be equable without sudden changes of level.'

In some cases a fall is almost impossible to obtain, as, for instance, at Southport, in Lancashire, where the ground is nearly a dead level. The fall there is about 1 in 5,000, and never exceeds 1 in 3,000. In such a case the drain would have to be cleaned either by locks or valves (flushing-gates) to retain a portion of the contents for a time, and then set them free suddenly in order to flush the next section, or by special arrangements, such as Field's flush-tank.

Access to Sewers.

It is of importance that to all sewers capable of being entered by a man, there should be an easy mode of access. Man-holes opening above, or, what is better, at the side, should be provided at such frequent intervals, that the sewers can be entered easily and inspected at all points. The man-holes are sometimes provided with an iron shutter to prevent the sewer air passing into the street, or by the side of the man-hole there may be a ventilating chamber.'

Calculation of Discharge from Sewers."

Several formulæ have been given, of which the following is the most simple :

V = 55 × (√D × 2F).

Then, if A = section area of current of fluid, VA = per minute.

discharge in cubic feet

To use this formula, the hydraulic mean depth when the sewage is flowing, and the amount of fall in feet per mile, must be first ascertained. The hydraulic mean depth is 4th the diameter in circular pipes; in pipes other than circular, it is the section area of current of fluid divided by the wetted perimeter. The wetted perimeter is that part of the circumference

' Mr. Baldwin Latham joins the sewers in man-holes, so that if one is blocked another may be used; the outlet being at the lower level.

* The following table, taken from Mr. Wicksteed, will be found useful:

Mr. Latham (Lectures on Sanitary Engineering, delivered to the Royal Engineers at Chatham) gives a table, of which the following is an extract :—

In this table the velocity in feet multiplied by the inclination equals the length of the sewer to which the calculation applies. For example, if the velocity is 6 feet per second in a pipe whose diameter is 4 inches, then 6 × 24 = 144 feet is the length of the

sewer.

of the pipe wetted by the fluid. tained, as the fall in 50 or 100 or per mile calculated (5,280 feet =

The fall in feet per mile is easily ob200 feet can be measured, and the fall 1 mile).

Movement of Air in the Sewers and Ventilation.

It seems certain that no brick sewer can be made air-tight; for on account of the numerous openings into houses, or from leakage through brickwork, or exit through gratings, man-holes, and ventilating shafts, the air of the tubes is in constant connection with the external air. There is generally, it is believed, a current of air with the stream of water if it be rapid. The tension of air in main sewers is seldom very different from that of the atmosphere, or if there be much difference equilibrium is quickly restored. In twenty-three observations on the air of a Liverpool sewer, it was found by Drs. Parkes and Burdon-Sanderson,' that in fifteen cases the tension was less in the sewer than in the atmosphere outside (i.e., the outside air had a tendency to pass in), and in eight cases the reverse; but on the average of the whole there was a slight indraught into the sewer. In the London sewers, on the other hand, Sanderson noticed an excess of pressure in the sewers.

If at any time there is a very rapid flow of water into a sewer, as in heavy rains, the air in the sewer must be displaced with great force, and possibly may force weak traps; but the pressure of air in the sewers is not appreciably affected by the rise of the tide in the case of seaboard towns.* The tide rises slowly, and the air is displaced so equably and gradually through the numerous apertures, that no movement can be detected. It is not possible, therefore, that it can force water-traps in good order, when there are sufficient ventilating apertures.

On the contrary, the blowing off of steam, or the discharge of air from an air-pump (as in some trade operations), greatly heightens the pressure, and might drive air into houses. So also the wind blowing on the mouth of an open sewer must force the air back with great force.

It is, therefore, important to protect the outfall mouth of the sewer against wind by means of a flap, and to prohibit steam or air being forced into sewers.

To how great an extent it is the openings into houses which thus reduce the tension of the air in main sewers is difficult to say, but there can be little doubt that a large effect is produced by houses which thus act as ventilating shafts.

When a sewer ends in a cul-de-sac at a high level, sewer gas will rise and press with some force; at least in one or two cases, the opening of such a cul-de-sac has been followed by so strong a rush of air as to show that there had been considerable tension. It is also highly probable, from the way in which houses standing at the more elevated parts of sewers, and communicating with them, are annoyed by the constant entrance of sewer air, while houses lower down escape, that some of the gases may rise to the higher levels.

That no sewer is air-tight is certain, but the openings through which the air escapes are often those we should least desire. It is therefore absolutely necessary to provide means of exit of foul and entrance of fresh air, and not to rely on accidental openings. The air of the sewer should

1 Report on the Sanitary Condition of Liverpool, 1870, p. 27.

2 Vide same Report, p. 21, for the case of Liverpool. Dr. Corfield's observation at Scarborough was confirmatory.