It is rightly considered that a scheme of town-sewerage cannot work satisfactorily without proper arrangements for flushing the sewers, the appliances for the purpose being placed at the head of all long sewers and at suitable intermediate positions. While town-sewers are laid at flatter gradients than house-drains, and the composition of the sewage is a great deal more complex, it is still necessary to have proper provision made for the flushing of house-drains. In many towns this work is now periodically done during the dry season by the sanitary authorities themselves, but in a well-devised system of house-drainage it is essential that some automatic appliance, regulated to discharge at suitable intervals, should be fixed at the head of the drain, for even if this is laid at a gradient that will produce a self-cleansing velocity, there are times when the flow of sewage is not sufficient to keep the solids from depositing, or to prevent their gradual accumulation and the consequent generation of gases. The opening of a half-inch tap at full bore is of no value whatever. What is required is to discharge through the drain a volume of water sufficiently large to thoroughly cleanse the whole of the interior surface that has been in contact with the sewage.

Mr. Rogers Field was the first sanitarian to put into practical shape the principles of automatic drain-flushing, and his self-acting flushing cisterns, fitted with a patent annular syphon, have been in general use for many years; the present pattern is shown in Fig. 405. The annular syphon is fitted into an iron tank. The longer limb of the syphon dips about 1/4 inch into the water in the lower tank or trapping-box, in which the water is kept at its proper level by a weir. The action is as follow: - When the water (fed from the inlet or tap) rises in the dome or Ml to the top of the lip on the longer limb of the syphon, it begins to flow over by a succession of drops at first, which dispel a sufficient quantity of air in the discharge-pipe and Ml to cauae the level of the liquid Within the bell to rise; this rise increases the overflow into the discharge-pipe and expels a further quantity of air, gradually forming a vacuum, which again causes a still larger rise and overflow, and thus brings the syphon into full action, when the tank is emptied with enormous rapidity.

Fig. 405.  Section of Field's Annular Syphon.

Fig. 405. -Section of Field's Annular Syphon.

Fig. 406 is an illustration of Adams's flushing syphon. When the- tank, in which the syphon is placed,commences to fill, water stands in both legs of the inverted syphon at the level a. This level dues not vary for a considerable time, because, although the tank is filling, the air imprisoned under the deep trap over the main pine can escape by the small pipe at the side, which also has a cap over it. But when the water rises high enough to seal the upper end of this small pipe, the air is retained. As it becomes compressed, it forces down the water in the left leg of the syphon until this leg is empty. A further increase of pressure causes a large bubble of air to escape up the right leg. This leg, it will be seen, is contracted at c, so that the babble is delivered up the centre of the pipe in one mass. Were it not for the restriction at C, it would be possible for the air to creep up the side of the pipe in small bubbles, when the effect would be less rapid and certain. As it is, as soon as the bubble turns the corner, it displaces a quantityy of water in the right leg. and when it has passed away the level of water suddenly falls from A to B The reduced head (b to c) can then no longer balance the air-pressure in the deep trap, and a rush of air clears out the syphon, to be immediately followed by all the water in the tank. To confine the requisite amount of air, a cap of sufficient depth is placed upon the vent-pipe shown, this pipe ensuring the absolute certainty of the apparatus giving free admission of air to the syphon both before and after its discharge, in the latter case completely braking the partial vacuum which other-wise would exist.

Fig. 406   Section of Adam's Flushing Syphon.

Fig. 406 - Section of Adam's Flushing Syphon.

Fig. 407 shows a Hushing syphon on a new principle, known as Merrill's patent. Instead of depending for its action on the gradually increasing pressure of the liquid in the tank driving out the air compressed in the syphon, Merrill's syphon depends on the action of a tipping bucket. When the water fed into the cister or tank reaches the inlet end of the short leg, the air in the syphon is locked up between the water in the tank and that in the outlet-trap, and consequently tlie water rises more slowly in the short leg than in the tank, the difference in level depending on the depth of the seal of the outlet-trap. When the feed supplied to the tank has produced a sufficient compression in the syphon, the air is gradually driven out through the trap, in proportion as the water rises in the tank and short leg. When the water reaches the bend or overtime point of the syphon, it overflows into the tipping bucket, which, when filled to its tipping point. overturns and throws out its contents into and through the outlet-trap, and thus produces a vacuum which permits the head of water to charge the syphon. The syphon is so constructed that the upper part can be lifted off, and the lower portion exposed for inspec-tion or cleansing.

These syphon-cisterns, when charged with clean water, may be placed either inside or outside the house in any convenient position; but if outside and above-ground, means must be taken to protect them from frost. To economize in the consumption of water, it is a good arrangement to feed them from the bath-waste, as this water is seldom greasy; and as thirty gallons are often discharged from a bath, a cistern to hold this quantity is a suitable size to adopt in such a position.