So far as the public sewer is concerned, it makes little difference what is the size of the house connection drain through the greater part of its course; but the junction with the sewer should, under no circumstances, where six-inch sewer-pipes are adopted, be more than four inches. I should even insist on four-inch connections with an eight-inch sewer. Through neglect, or by reason of improper management, many kinds of rubbish find their way into house-drains; and a four-inch opening will admit as many of these into the sewer as it will be able to carry away. If, by reason of bad construction or neglect, an obstruction is to be caused at any point, it should be in the drain, which the person responsible for it must cleanse or repair. The grease-trap referred to above may be any form of reservoir which will retain the flow from the kitchen sink until it has time to cool, when its grease will be solidified, and will float at the surface. The outlet from this trap should be at such a distance below the surface of the water, that there will be no danger of its floating matter passing in with the discharge. A very simple device for this purpose is shown in Figure 5. From a trap of this sort the flow is constant whenever additions are made to its contents. [Illustration: FIG. 5.--GREASE-TRAP. I, Inlet; V, ventilator; O, outlet.] [Illustration: FIG. 6.--FIELD'S FLUSH-TANK. A, Receiver; B, grating; C, ventilator; D, siphon; F, entrance to drain; I, delivery from sink.] Figure 6 shows the invention of an English engineer, Mr. Rogers Field, which has the effect of retaining all of the outflow from the kitchen sink until it is entirely filled,--say thirty gallons. When filled, any sudden addition of a few quarts of water, as from the emptying of a dish-pan, brings into action a siphon whose entrance is near the bottom of the tank; and this siphon rapidly discharges all of the contents above its mouth in a flow having sufficient force to carry forward not only any solid matters which it may contain, but also any ordinary obstructing accumulations in the drain below. The soil-pipe, carrying the discharge of water-closets, should not be delivered into the flush-tank, but at a point farther down the drain, so that any solid matter it may deposit shall be swept forward by the next action of the flush-tank. The more often the flush-tank is filled, and the greater the proportion of its water to its impurities, the more efficient will be its action. Therefore the slop closet waste leading from the upper story, and even the outlet pipes of bathing-tubs, may with advantage be delivered into it.