stone as to prevent their floating when pumped empty for inspection or cleaning, and should be absolutely tight. The inverted siphon is made sometimes to slope from both ends to a point near mid-length, sometimes with a vertical drop at one end, sometimes at both ends. The first should be adopted only when the siphon is sufficiently large to permit the entrance of a man. When not of such a size, it should be straight from end to end. This will usually require a shaft at one end, sometimes at each, which may also serve as a manhole. It is in most cases advisable to place a catch-basin at the foot of such a shaft, although in place of this a basin in the bottom of an enlargement of the sewer just above the siphon is sometimes employed.

Unless a siphon under water is of large size and in tunnel or laid in a trench in a rocky bottom, it should be protected from undermining by currents, or movement by shifting bottoms or channels. This protection is usually afforded by driving a row of sheet-piling on each side of the pipe, the space between these being in most cases excavated and filled with concrete. The softer the material in the bottom and the stronger the currents the deeper the sheeting should be driven. If the bottom is too hard to permit of driving sheeting, large stone rip-rap may be placed on both sides and over the siphon.

A sewer must sometimes pass either under or over an obstruction such as a water-main, another sewer, etc.—and if under, this must generally be done by an inverted siphon. Such a siphon is usually a few feet in length only and under but little head. A manhole should be placed over or near it when the sewer is 24 inches or more in diameter, since it will probably need more frequent cleaning than the other parts of the line. If the sewer is less than 24 inches diameter, a manhole should be placed at each end of the siphon (which is preferably straight from end to end).

Sub-drains are placed either directly beneath the sewer or at one side of the trench. When there are no artificial foundations under the sewer the latter position is to be preferred, but is in some instances much more difficult and expensive, particularly in quicksand. The sub-drain should be surrounded with

broken stone or clean gravel, varying preferably from the size of a hickory-nut to that of a pea. There should be at least 3 inches of this under the drain and 6 inches at its sides and top. In quicksand or similar material these dimensions should be increased 50 to 100 per cent. This stone should be well compacted to prevent future settlement. The joints of the drain should be slightly open and a 5- or 6-inch strip of cheese-cloth or burlap wrapped around the pipe at the joint to keep out the dirt. Or, if bell-and-spigot pipe is used, a piece of jute may be calked loosely into the joint for this purpose.

When the sewer is laid directly over this there may be danger of a settlement of the same and of leakage resulting. For this reason the sub-drain may be laid at one side of the trench when the soil is firm. In quick or running sand this is practically impossible unless the trench is very wide or unless close sheathing be driven on each side of the sub-trench and carried below its bottom; such sheathing not to be removed after the sub-drain is laid. It would usually be better and cheaper than this to lay the sub-drain in the center of the trench (which must of course be close-sheathed in quicksand), and on the stone filling, when levelled off, to place a continuous platform on which to lay the sewer. A still better construction in any but firm soils is to lay a pipe sewer in concrete. Such construction is shown in Fig. 57. Where a foundation is necessary for the sewer, the sub-drain construction is easily arranged.

The sub-drain should be laid to grade as carefully as the sewer itself. It is seldom that a sub-drain can be so arranged that inspection can be made of it, and therefore perfectly straight alignment is not necessary; but there should be no sharp angles in its line, which might cause obstructions or interfere with the future cleaning of it. If cellars and basements are to be connected with this drain, Y branches should be inserted to permit of such connections, and should be covered similarly to the separate-sewer branches.

Rising House-connections. When separate or separate or combined sewers are placed with their tops more than 4 or 5 feet lower than the average cellar depth in that locality, it is advisable to place a

rising house-connection above each branch, bringing it to within 3 to 5 feet of the average levels of the cellar bottoms, but stopping at least 7 or 8 feet from the surface. This is to avoid compelling each householder along the line to dig down to a deep sewer branch in order to make a connection. These rising connections are built while the sewer-trench is open, and are covered at the top with a cap or cover similar to house-branches. They should not merely rest in the branch, but a foundation of concrete or brick masonry should support each. The vertical pipes should be held in place during back-filling, as by stakes driven into the bank. In the case of a rock cut, or where the banks are not

firm, the rising connection may be inclosed by a vertical trough of planks, between which and the pipe earth is packed, this trough being held firmly in place until the trench is filled and tamped. If the banks are liable to cave, sheathing should be driven at each such connection, and neither it nor the braces removed when the trench is filled.

Foundations. A sewer in soft soil, like any other structure, requires a foundation. Since the weight is not comparatively great, the service of the foundation is more often to distribute the pressure and prevent local settling or heaving than to prevent the subsidence of the sewer as a whole. This purpose is usually achieved by use of a cradle, or a platform of plank,