may be used with more or less success for the disinfection of sewage. We will here merely mention a few of the best disinfectants, whether as applied to a system of sewers or a mass of liquid sewage.

Perchloride of Iron-1 gallon of perchloride suffices for 15,000 gallons of sewage.

Sulphate of Iron-1 lb. dissolved in 8 gallons of water for 1000 gallons of sewage. Carbolic Acid-3 gallons of the dilute acid to 1000 gallons of sewage.

Süvern's Deodorant-It is made thus: A bushel and a half of good quicklime is put in a cask, slaked, and 10 lbs. of coal tar thoroughly mixed with it; to this 15 lbs. of magnesium chloride dissolved in hot water is added, and finally additional hot water is poured in sufficient to make a mass liquid enough to drop from a stick inserted in it and then pulled out. The magnesium chloride forms deliquescent calcium chloride, and prevents the caking of the deodorant and the adherence to pipes.

Any one of the above used in proper and sufficient quantity is an effectual disinfectant and deodoriser of sewage.

Sewage as a Cause of Disease.-That the sewer gas does occasionally give rise to symptoms belonging to no known disease, to symptoms, in short, of poisoning, is without doubt. For example, Dr. Handfield Jones relates some remarkable cases in the "Medical Times and Gazette," 1871, vol. ii. p. 9, clearly attributable to sewer emanations. The first of these, a man, aged forty-nine, was taken with giddiness and shortness of breath whilst at work in a sewer which stank very badly. He complained of feeling numb all over, and there was oedema in the feet and legs; the abdomen was tumid, and the urine was albuminous. He was ill about three weeks.

The second case was a man aged forty-nine. In passing a gullyhole he was conscious of a most disagreeable odour. In half an hour he was taken with severe vomiting, which lasted all day. He was admitted into the hospital, and suffered from cramp, sickness, dimness of sight, &c. There was no diarrhoea nor other evident cause.

Dr. Handfield Jones also quotes a case from the Sydenham Society's Year-Book, in which a ground labourer, aged forty, after working for three hours in a sewer, was compelled to leave off on account of the horribly stinking atmosphere. He was ill for several days, and suffered from languor, anorexia, and sleeplessness, with slight nocturnal delirium. There was no fever, but slight jaundice, and on the eighth day hæmorrhage from the nares and pharynx supervened.

"In conclusion, let me observe, that as these were cases of acute poisoning by sewer emanations, so undoubtedly cases of chronic and slight poisoning by the same agent are vastly frequent."

Some

As the germs of typhoid fever exist in some sewage in incredible number, it occasionally must happen that this disease is propagated by irrigation. Such an instance appears to have happened in Ecton, a parish in which are situated the Northamptonshire Irrigation Meadows. Dr. Buchanan, who inquired into it, thus summarises the case: "The facts of the Ecton occurrences, therefore, are now pretty clear. In the early days of July last, ten people are working on a meadow through which runs a brook containing Northampton sewage, of which a part is formed by the excrement of patients with enteric fever. at least of the ten people employed in this meadow drink of the brook, in ignorance of the nature of its contents. Almost all the workers become sick. Two of them, who cannot be followed, get diarrhoea; a third gets a protracted diarrhoea, which bears resemblance to that of enteric fever; a fourth and a fifth get distinct enteric fever, one of them ten days after the other. Of the two latter, first one and then the other goes home and infects the common privy, and doubtless the well of the yard in which their house stands. Other people living in that yard, themselves having nothing to do with Northampton sewage, begin to sicken with enteric fever two or three weeks after this introduction of the disease among them, and fall ill one after the other of the same fever, until fourteen out of the eighteen residents there have been attacked Meanwhile, among the other 600 residents, there is no case of fever, except a solitary and presumably imported one. On the other hard, out of 120 people at work upon the sewage farm itself, there is no single case that can be affirmed to be fever, and the only case of illhess that can be heard of is a case of diarrhoea."

Such isolated outbreaks are to be attributed to carelessness and want of knowledge. They are certainly exceptional-more so, indeed. than from theory one would imagine. There appears to be danger from drinking the water flowing from, but little in walking over, an irrigated soil.

The manufactories of manure at Bely near Paris, appear to be healthy, nor is there a history of the propagation of fever in ary of the irrigation processes, save and except the one quoted above.

Sewage pent up in sewers, cesspools, or per colating into and infecting the soil and water near dwelling-houses, is certainly most

of cholera, typhoid and scarlet fever, &c.; but sewage in manure works, undergoing chemical processes, and sewage in the open air, remote from houses, irrigating fields, is only in rare instances hurtful to health, although at the same time it may be an annoyance, and therefore a nuisance.

The bad effects of human sewage on cattle appear to be nil; they grow and fatten on the most sewage-sodden soil, nor has there been an increase, as was feared, of entozoa in their bodies.

Sewers, Drains, &c.

Sewers.-The word "sewer" in its most extended sense is used to signify a channel (which is generally covered) for the reception and removal of impure and refuse liquids holding solid matter in suspension derived from two or more habitations. The Public Health Act distinguishes for the purpose of the Act the word "sewer" and "drain" as follows:

"Drain' means any drain of and used for the drainage of one building only, or premises within the same curtilage, and made merely for the purpose of communicating therefrom with a cesspool or other like receptacle for drainage, or with a sewer into which the drainage of two or more buildings or premises occupied by different persons is conveyed.

666 'Sewer' 'includes sewers and drains of every description, except drains to which the word 'drain' interpreted as aforesaid applies." It hence follows that it is not the size or shape of a channel which determines whether it be a sewer or not, but its office, and whether it is used by more than one house.

The most ancient sewers-as, for instance, the cloaca maxima of Rome-were built to carry off rain and subsoil water, and hence they terminated in the nearest watercourse. They afterwards were made channels for the removal of excreta, because it was the easiest and most obvious appliance.

The old sewers of modern towns were mostly open and extremely ill-constructed. Take, for example, the ancient system at Paris, which is thus described by Tardieu :

The Seine, the Meuilmontant, and the Bièvre have ever been the great outlets for Paris. It was towards these three lines of drainage that the ancient inhabitants directed their slops and surface-water, by means of gutters carried across the fields which surrounded the groups of houses forming the town. At a later period a part of the fosses des enceintes of Philip Augustus and Charles VI. also received the filthy waters of Paris. All the sewers, open, and for the most part badly levelled too, rapidly filled with refuse and stagnant water, and infected the air. They

were little by little cleansed and improved. The worst were suppressed; the bottoms and sides of others were built in masonry; and lastly, they thought of covering them with flags or stones."

A great change has taken place since those times, for Paris is now one of the most completely sewered cities in the world. Underneath all the principal streets, not only sewers but subways are constructed, the tributary mains are named, and the house drains numbered. "The total length of these sewers is about 170,000 metres, which added to the 290,000 metres in course of construction, and to the 80,000 metres which will perhaps be at some future "time opened, may bring the number to 135 leagues as the total length of the subterraneous canals of Paris."(TARDIEU, 1862.)

Human excrement to a certain extent exists in all sewers, but in some towns there have been attempts made to keep it entirely out of the sewers. For instance, in Paris the system of fosses permanentes and fosses mobiles diverts the bulk of the excreta away from the ordinary channels. It would also appear, from the discovery of enormous pits in Rome by Dr. Parker, that the main portion of the Roman sewage was collected in these pits, and did not find its way into the subterraneous conduits.

It is extremely important ever to remember that sewers and sewage not containing human excreta, and not communicating with waterclosets, &c., are quite as offensive as if there were communication with privies and waterclosets; still, as they do not contain the excretions of man, it necessarily follows that the germs of those contagious diseases which are found in typhoid, &c., are probably absent, and that any disease which such sewers may give rise to would be merely from the effect of putrid emanations.

Construction of Sewers. Although the original idea of a sewer was the natural outcome of draining the subsoil, it is dangerous to construct the sewer of pervious materials, under the idea of making it a drain as well; sewers must be impervious. The larger kind are usually made of bricks specially moulded to radii, and set in cement; the smaller are constructed with socketed pipes, the joints made water-tight with clay puddle, and the whole generally laid on a bed of concrete to prevent sinking of any portion of the track.

Shape of Sewers.-All main sewers of large towns should be oval; the section preferred is egg-shaped, the smaller end of the oval pointing downwards. The advantage of this particular shape is twofold: the sewers are stronger and more economical; and, besides,

there is greater efficiency, for when the water is small in amount, the narrowness of the lower part gives a greater hydraulic depth, and when the body of water is increased, more capacity is obtained.

The smaller sewers or sewer drains are best constructed of round pipes. It is difficult to obtain accurate ovals with earthenware pipes that is, so that each pipe will have exactly the same section-and in smaller drains the difference in shape is not practically of importance.

The old square brick drains are the worst, as to construction, efficiency, and economy, that can well be conceived. There is a maximum of friction and porosity; in addition to which, the rats burrow in them, so that the sewage may find its way into wells a considerable distance from the drains.

The Size of Sewers.-This must greatly depend upon whether they are intended to take storm-water as well as sewage, or not. The best way is to have separate brick drains for the surface and subsoil water, the brick drains being superficial and the sewers deep.

"It is calculated that a main sewer intended to receive all the sewage of a thicklypopulated square quarter of a mile, with a water-supply of 20 gallons a head, and also the rainfall of the same surface, would only actually require for these purposes a sectional area of 4 feet square, but that practically, in order to provide for sudden storms, this size would have to be at least doubled."

A size much less than this would suffice if there were separate channels for the rainfall. Some of the sizes in actual use are as follows:

"In London, in the streets the brick sewers are from 4 feet 6 inches by 2 feet 6 inches to 9 feet 6 inches by 12 feet, the latter being the size of the largest mains; in the courts and alleys the sizes are from 3 feet by 2 feet 2 inches to 4 feet by 2 feet 4 inches. In Dover the main valley sewer is 4 feet 6 inches by 3 feet. In Salisbury the new sewers are about the same size, and the mains are so constructed that the subsoil-water percolates into them freely. In Bristol the main outfall is 5 feet by 4 feet 6 inches, and the mains vary from this size to 2 feet by 1 foot 6 inches; and it will be found practically, that where the drainsewer system is still carried on, the minimum size for the mains will be about 3 feet 6 inches by 2 feet. This size would allow a man to creep through the sewer easily, if necessary." When we have towns sewered with impervious glazed stoneware pipes, we find the size at Dover to vary from 6-inch house pipes to 18-inch sewers, and opening into brick mains. At Rugby the pipes are only 6 inches and the

| main outfall 2 feet in diameter. At Stratfordon-Avon the smallest street sewer is a 9-inch pipe, but then the sewer collects the stormwater. It is considered that there is no necessity to make sewers either so large that men may creep through them or to take the whole of the storm-water. The small pipes at Rugby answer just as well as the enormous brick sewers of other places. The stormwaters will always find their way over the surface in natural or artificial drains to the

nearest watercourse.

Sewers should be laid in as straight lines as possible, and placed in a bed which will not be likely to sink. They are laid generally on concrete. The gradients must be true throughout. If curves are necessary, the radius of the curve should be not less than ten times the cross-sectional diameter of the sewer. The fall for street drains is usually from 1 in 244 to 1 in 784. The flow through a sewer of any size should not be under 2 feet per second nor over 4 feet 6 inches. It is a mistake to imagine that the fall cannot be too great. The following table of Mr. Wecksteed's may be useful:

Diameter of the Sewer.

·

4

240

A

section area.

The hydraulic mean depth is one-fourth the diameter if the pipe is running full; if the pipe is not full, it is the section area divided by that part of the circle of the pipe wetted by the fluid, which is called the wetted perimeter.

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 Fig. 81. sewage at the junction; what is called a bell-mouth is formed with a ventilating-shaft up to the roadway (fig. 81'.

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, &c. When from any cause a part of a sewer is blocked, a man descends the manhole, another one lowers a candle down at the nearest point of access from the manhole, and the deposit is removed by means of proper instruments until the light of the candle or lamp is seen.

This is an expensive method of cleansing sewers; for instance, the cost of removing deposit from the tide-locked and stagnant sewers in London formerly amounted to a sum of about £30,000 per annum.-(BAZALGETTE) A far better way is to provide a flushing apparatus, which in its simplest form consists of a dam of any kind, ponding up the water; this is then suddenly removed. The power of water in this way is surprising. In an experiment with a flushing-gate 4 feet high, the quantity of water ponded up for one flush was 26,665 cubic feet; three flushes carried brickbats 1300 feet. "In an instance

removed by flushing, it was calculated that as the whole cost of removing it by hand labour would have been £2387, while the cost of putting up the inside apparatus and flushing-gate was £1203, and the cost of men's time £644, 12s. 7d., there was thus a saving of £456 to the commission; besides the fact, that on account of the side-entrances the pavement would no longer require to be taken up as before, and the apparatus would remain to be used when required." It certainly is not well to wait until there is a deposit before flushing; sewers should be flushed and disinfected regularly, and in that way much expense may be saved as well as danger to the public health avoided, since, where no accumulation of filth is permitted to take place, foul gas is not generated.

Ventilation, Deodorisation, and Disinfection of Sewers. -All sewers should be ventilated, not only on account of bad odours, but because any change of temperature in the sewer liquid either expands or contracts the sewer air. If steam or hot refuse-water is thrown into a sewer the air expands, and may under certain circumstances force the traps. For example, let a b (fig. 82) be the commence

resist.

The same thing as suggested by Mr Latham can be shown by the following neat experiment: Bend a glass tube

Fig. 85.

in the shape of a trap (fig. 83), and fix it carefully into a well-fitting cork to a flask a; put a little tincture of litmus or coloured water into the bend b; now, by taking hold of the flask a, the mere heat of the hand

will jerk the liquid in b out of the trap.

With regard to the distance between the ventilating-shafts, they should be placed about every 300 feet, or at all events all the manholes should be furnished with them.

Fig. 84 will give an idea of the ventilatingchambers furnished to some sewers. Those sewers which are likely to be ponded up by

shafts, the sectional area of which should be at least half as great as the sewers.

The shafts and chambers should each be furnished with trays of charcoal, and the