If two sets of conduits are required, one for the flow of sewage and another for surface water, no certain saving in first cost would result. In fact, a combined system would often cost less than the two. Moreover, two sets of conduits under our streets would lead to much trouble in arranging the details, such as the proper levels, and which pipe should pass over the other where intersecting at street corners. Trouble has also arisen where such a double system existed, in the service pipes of one system being interfered with by the main of the other system in trying to construct such services on proper slopes between the houses and the mains. Mistakes are likely to occur, also, in making the private connections to the mains, by entering the wrong main, which class of mistakes are difficult to avoid in such a complication, where the administrative officers and foremen in charge of the work are subject to change in their personnel.

Large sewers are now constructed so as to be more self-cleansing than formerly, by contracting their section at the bottom, so that the stream flows over an invert in the large sewer of a similar shape to the lower half of a small pipe.

Fig. 109, which is copied from Baldwin Latham's work, shows the improvements made in this respect.

Since it is not proper to discharge crude sewage into small streams, all towns that are remote from the sea or very large rivers should provide for the purification of their sewage, to prevent it from becoming a nuisance.

The system of disposal of sewage by irrigation of the soil has been referred to above, as the most efficient, most economical, and least harmful of the processes hitherto tried for its purification, at all places where a suitable opportunity can be found, within a reasonable distance, for its development. Its application has hitherto been limited in this country to a very few cases, and even in these the experiment has been on a small scale, e. g., the hospitals for the insane at Worcester and at Danvers, Mass., and at Augusta, Me. This method can never be pursued with any prospect of economic results as to the crops produced on the sewage farms, unless the collection of the sewage should be made without much or any rain-water. The exclusion of the surface water becomes more imperative in America than in England for the reason that American towns which have a public water-supply consume a much larger quantity of water per capita than European ones, by means of which the sewage is more largely diluted, and therefore of less value as a manure per ton. The distribution of water in most of our towns is not only liberal but lavish, being aggravated by a reckless waste which the municipal authorities

The enor

have hitherto taken no efficient steps to check in most cases. mous expenditure for fixed plant in water-works which has taken place in cities like New York, Philadelphia, and Boston, with the actual quantities of water now distributed, would, if properly husbanded, supply liberally fully twice the present population in these cities. No large city of this class can ever dispose of their sewage upon the land with a hope to produce crops by irrigation, till some way shall be provided for checking this prodigal waste of water and thus rendering the sewage less dilute in its character. The sewage farm at Barking, on the Thames, has a small portion of the sewage of London to dispose of, and its profits have been reduced to a microscopic amount, if not a negative quantity, by this superabundance of water. Yet the quantity of water distributed per capita of population in London is far less than that enjoyed and wasted by the large American cities. The following table is made up from "Croes' Statistical Tables," giving figures for 1880, for eleven prominent American towns, while those for London are taken from official reports quoted at London for the same year, in Engineering (vol. xxx., p. 195).

Consumption of Water in 1880, in various American Cities, compared with London.

But it is the opinion of all sanitarians, that for all inland towns having no opportunity to discharge their sewage into the sea or some large river in a way which would create no nuisance, the purification of their sewage becomes imperative. For this purpose irrigation is the only rational method of treatment, and should be adopted, even if the crops produced are not a source of profit. If the notion of profit is abandoned as unattainable, the process may be much simplified by allotting more sewage to the

'The consumption in London is doubtless reported here in imperial gallons of 288 cubic inches, while the American cities reckon by wine gallons of 233 inches only. This would require an addition of 25 per cent. to the figures given for London to make the comparison fair; but on the other hand the consumption in London is reported for the month of July only, which is doubtless larger than the average for the year, perhaps by an equivalent ratio. Moreover, the report of the London official gives only the population actually served with water in their houses, which we suppose to be a smaller ratio of the whole population than is the fact in American cities.

area treated than the crops could be expected to profitably use, and removing the water by under-drains after it has filtered through the soil. Such a process cannot be kept up continuously with success. There must be intermission by means of two or more fields for alternate treatment, giving each field such period of rest as will enable the water with which it is gorged to soak down and admit the air to the pores of the soil. It is this very air in the pores that does the work of chemical purification by means of its oxygen, which process is more important and more efficient for the purification of the effluent water than the straining or mechanical filtration.

This process is perfectly applicable to villages and small towns, and requires less expenditure for its maintenance than is generally supposed. It is not entirely automatic, however, and needs frequent attention in diverting the flow from one plot to another, as often as the soil becomes saturated. Such attention, however, would not be an onerous tax upon a village of a thousand inhabitants or more.

IRRIGATION BELOW THE SURFACE.

For all small villages or collections of houses, as well as for single houses in the country where the land is not entirely flat, a distribution of the sewage can be made, about a foot below the surface, by porous tiles, which has been tried both in England and this country with success; and the process is nearly automatic when the apparatus is properly prepared, requiring attention only at long intervals. The requisites for this system are as follows:

First.-Land adapted to grass, nearly level or gently sloping, at the rate of one-fourth of an acre for a single family, or an acre for a combination of eight to ten families, if provided with a constant water-supply under pressure. If the water-supply is limited to what may be pumped by hand, one-half of the above area will be ample.

Second. The highest part of the land devoted to the purpose should be at least five feet below the level of the top of the drain where it leaves the house.

Third. The soil should be thoroughly under-drained, if not resting on a dry and porous subsoil by nature. Under-drains are often needed in clayey or retentive soils, and should be laid at least four or five feet below the surface, at intervals of about twelve feet, with a free outfall.

Fourth. The land should be graded, unless tolerably smooth before hand, so as to avoid sudden inequalities. A surface that is adapted to smooth mowing by hand is good enough for the purpose.

Fifth-The soil must be entirely free from roots of trees and shrubs. These would choke the pipes in a few weeks.

The cost of the work will vary with the local conditions. It can be laid out by any intelligent mechanic, with an ordinary spirit level and straightedge twenty feet long, though if on a large scale, requiring an acre of land or more to be treated, an engineer's level would be a convenience.

If house drainage is conducted directly into porous tiles laid under the surface, the fluid parts will escape at every joint, while the solid matter is apt to cling to the interior and gradually fill them, till they become practically useless, unless taken up and cleaned.

In order to avoid this result it is advisable to provide a tank or tight cesspool where all the sewage is arrested for a while, during which time

the solid matters become macerated and finely divided by fermentation, before entering the distributing pipes. Moreover, if such a tank be allowed to overflow constantly into the porous pipes by a dribbling discharge, they would become choked after a while even then. In order to keep them free, the flow must be intermittent and take place with such a rush as to fill the whole system of distributing pipes at once, and brush away slight obstructions which may have been left by former discharges.

There are two methods of obtaining this result: First, by providing a stop-gate in the outlet-pipe where it leaves the tank, to be opened by hand when the tank is full and closed again when empty. Second, by providing a siphon or float to discharge the tank automatically whenever filled.

Of course the latter method is far more satisfactory, if made reliable, but it is somewhat more costly, and the siphon, as heretofore constructed, is not always reliable, i.e., the apparatus for setting the siphon in action. does not always work as expected.

Field's siphon has generally been used for this purpose, and works well with pure water; but if used with sewage on a small scale, it is liable to miss, unless the cup into which it discharges is occasionally brushed out. When used on a large scale, as for a combination of several houses, this difficulty disappears.

Several other devices have been used for starting a siphon. In one a tumbler tank is placed in the upper part of the flush tank, which upsets when filled, turning on brass trunnions, and righting itself at once when empty. A tumbler tank of two gallons capacity will start a siphon of two inches diameter, if the lower or discharging end dips in water as soon as the flow through it begins. It is important that it should not so dip when no water is flowing through it, for in that case the flush tank would never be filled again after once discharging; but it can be so arranged that a very slight flow would seal the discharging end.

The device patented by Mr. Field was to accomplish this result.

The tumbler tank accomplishes the result well enough when filled with pure water, but if used for sewage, the solid matters are apt to adhere more or less to its sides and thereby destroy its poise, on which its certainty of action depends.

Other devices have sought to accomplish the same end by a float actuating a valve in the bottom of the flush tank, or by a bucket in a side chamber which can be filled by the overflow of the tank, and thereby becomes heavy enough to open the valve, while a small leak in the bottom allows the valve to raise it again after the tank is empty.

All of these contrivances are subject to failure from wear of moving parts except Field's siphon, which it is hoped may be perfected still further. Fig. 110 shows the two tanks and the Field siphon as modified by the writer, the changes being at least a partial remedy for the imperfections complained of.

If the siphon were to be applied as the outlet of the same tank which receives the sewage for maceration, etc., it would often become choked by solid matter. It is therefore best to provide a second tank between this and the siphon. The first is called the "settling basin" and the latter the "flush tank" on Fig. 110, for the last is alternately filled and emptied, while the first remains always full.

Siphons are often constructed inside of the flush tank, but it is better to make them accessible at all times by placing them outside, as here shown.