was designed to admit to the house-sewers roof-water and drainage from yards, and this method is still followed there to a considerable extent. In the United States the majority of separate systems are not supposed to receive this water. It is argued by advocates of the former practice that the householder should not be required to construct two connections, one for house-sewage and one for rain-water. But the last can be conveniently discharged into the gutter, except in the case of buildings covering a large area, when the cost of the extra drain would be relatively inappreciable.

Another argument for the admission of roof-water is that it is beneficial in flushing the sewer. If it is admitted only at and near the dead ends it will usually be advantageous, but it should not be thought to take the place of all other flushing. The sewers are most likely to need flushing at dry seasons, and this must then be done by hand or otherwise. There is a danger that the presence of these roof-connections will give a false idea that the flushing requirements have been entirely met.

If roof-water is admitted to small sewers throughout their length there is great probability of its gorging the pipes and backing up into connected basements and cellars. In Mount Vernon, N. Y., in 1892 great damage was caused in this way and all roof-drains were at once disconnected; and many similar instances might be cited.

Since the danger is so imminent and the benefits contributed at such uncertain intervals, most American engineers do not advise the admission of roof-water to small sewers.

Sewers are sometimes flushed by connecting their upper ends with convenient streams, or artificial channels filled from such streams, the water being admitted periodically by gates: as at Bern, Wurzburg, Innsbruck, Freiburg, Breslau, Munich, and other cities of Europe; also at Newton, Mass.

Reservoirs fed by streams or springs are used in Munich,

Cologne, Wiesbaden, Frankfurt, Stuttgart, and other cities. At the first-mentioned place large underground reservoirs, one of which is 6 feet 6 inches by 4 feet 7 inches and extends along two blocks, are filled from the Isar River.

Tides are sometimes made use of for this purpose, the water being allowed to rise in the sewer at high tide and being held there by gates until the low tide, when it is released. Ordinarily only the lower reach of the outlet sewer can be thus flushed. A better method in some cases is to hold the water after high tide in a basin from which it is rapidly discharged at low tide into the sewers to be flushed.

As in the case of Milwaukee, already cited, and of Bremen, the flushing-water may be pumped from a lake or river directly to the sewer. This is of course applicable within the limits of economy to very large sewers only, or to a system where a number of dead ends can be reached by a comparatively short line of water pipe.

The water for flushing is sometimes taken from the ocean or other body of salt water; but the salts are thought to decompose the sewage, giving rise to gases and deposits of matter rendered insoluble, and are corroding to any metalwork in the sewers. Hence its use is not advised by most authorities.

ART. 27. APPLIANCES FOR FLUSHING.

Automatic flush-tanks are in use in a large number of separate systems, but are seldom used for flushing combined or storm-water sewers, owing to the enormous quantities of water needed for that purpose. There have been a great number of devices invented for flushing. Most of those at present used in any considerable numbers are siphons in principle, so arranged that a tank in which they are set may fill gradually up to a certain point, when its contents are dis

charged rapidly into the sewer. The tanks are made to contain at the time of discharge from 150 to 600 or even 1200 gallons for 6- to 10-inch pipe sewers. For larger sewers larger quantities are provided. The smaller quantities are of little use. No tank should discharge less than 250 gallons at a time into a 6-inch pipe, and correspondingly larger amounts into larger sewers. 500 to 800 gallons discharged into an 8-inch pipe once in 24 hours would be more beneficial than half of that amount at each of three or four discharges during the same time. It is probable, however, that in sewers calculated for a velocity exceeding 5 feet per second equal efficiency may be obtained with quantities less than those stated.

The tanks should, of course, be water tight. They are usually built of brick plastered on both the inside and the out, but might be made of wood or of iron. They should be so built and arranged that the water may have the greatest permissible head above the sewer when discharging. (For details see Art. 47.)

The water may be conveniently admitted to the tank through a half-inch or smaller stop-cock connected with the street-main by a supply-pipe passing through the tank-wall. This cock is continually left sufficiently open to cause the tank to fill and discharge at desired intervals. If the water is inclined to be muddy at times the use of too large a supplypipe will result in the choking of it by sedimentation. It should be of such a size that the quantity to be used in the tank will pass through it with a velocity of 2 feet per second

or more.

The discharge-pipe of the tank should be at least as large as the sewer. It would be better to have it a size or two larger and bell-mouthed at the end, but this is seldom done.

The automatic flushing appliances most in use in the United States are further referred to in Chapter VIII. They

are, most of them, covered by patent, and the prices range upward from about $12 for a tank to discharge 150 gallons through a 5-inch pipe.

Where automatic flush-tanks are not used some engineers have built into manholes at dead ends 2-inch to 4-inch pipes connected with adjacent water-mains and provided with gatevalves, as at Mount Vernon, N. Y., and Newton, Mass. This is probably the most convenient method of hand-flushing and the cheapest to operate. The cost at Mount Vernon was about $40 for each 4-inch branch and connection.

There are numerous methods of flushing by hose, by water-tanks, etc., many of which are described in Part III.

In flushing by rain-water no special appliances are ordinarily used, the roofs and sometimes the yards being connected in the ordinary way with the sewer.

Special methods involving pumping, some instances of which have been referred to, need no description, since the details will vary with each case.

ART. 28. NECESSITY FOR VENTILATION.

In every sewer there is a space above the sewage filled with air, and this air, it is evident, will generally be far from pure unless kept in motion and frequently renewed. The odor accompanying all sewage, even when there is no decomposition proceeding in the sewer, is communicated to this air; there will frequently be given off some gases due to putrefaction; and it is possible that malefic germs may escape in vapor from the sewage or from deposits in the sewer, to be carried along by the air-currents. This air probably is seldom motionless. It is influenced by the sewage to move down the sewer; it is warmer in winter and often in summer than the outside air, which condition occasions motion when there is communication between the two; it is driven out of or along

the sewer by sudden inflows of sewage from house-connections or branches and sucked in by decrease in the volume of flow; near the outlet the direction and force of the wind affect it, driving it up the sewer or sucking it out; last, and most important, it passes into empty or partly empty house-connections and into proximity to, if not into the air of, connected residences. Herein lies the danger. There is no There is no "sewergas " which is deadly to human life, but it is known that air which has been confined in contact with decomposing sewage is charged with "an. ever-varying mixture of gases; and of those that are deleterious the more prominent are sulphuretted hydrogen, sulphide of ammonium, and caburetted hydrogen; while ammonia, carbonic acid, and occasionally carbonic oxide derived from leakage of illuminating-gas into sewers are present in more or less large proportions. (W. P. Gerhard,

The least that can be said of these is that they lessen the vitality and prepare the way for easy conquest by diseases that might otherwise obtain no hold upon the system; they should therefore be excluded from all occupied buildings. The danger due to impure air in dwellings has led the New York Board of Health to conclude that “ 40% of all deaths are caused by breathing impure air." Playfair asserts that in modern hygiene "nothing is more conclusively shown than the fact that vitiated atmospheres are the most fruitful sources of disease." Death rates have been "reduced in children's hospitals from 50% to 5% by improved ventilation."

While the vitiation referred to in these quotations is not that of sewer-air exclusively, this is included among the causes of it and produces the same effect. Unfortunately the most numerous and fruitful sources of the gases are found, not in the sewer, but in the house-connections or soil-pipes, and consequently not directly under the control of the authorities.

The methods necessary to prevent danger from