height. Another templet 24 inches diameter is fastened at the level of the top of the brick-work, its centre vertically above that of the bottom templet.. Cords are strung from the edge of the top templet to the top of the vertical part of the brick wall, spaced about 2 feet apart around its circumference, and the brick laid to these. An experienced manhole mason, however, can build almost as symmetrical a manhole by eye only, and more quickly than if strings are used.

When the wall is about 2 or 3 feet high the benches and channels of the bottom may be constructed. It is well to lay plank in the bottom over the channels temporarily, to keep mortar and dirt out of them and out of the sewer during construction, as well as to hold the brick and mortar being used. The first step should be placed about 18 inches or 2 feet above the bench. When the wall is about 4 feet high four piles of brick, each 8 inches square and about 3 feet high, may be made on the bottom of the manhole and a platform of short loose plank be placed on these, entirely filling the manhole. This holds the mason, brick, and mortar until another 3 feet are built, when a second platform is similarly placed 3 feet higher. These are of course removed when the brick-work is completed.

The brick in a manhole may be laid as all headers, all stretchers, all on end with their edges exposed, or a combination of any two or all of these. Bats may be used in large or small proportion or not at all. A strong manhole can be built by using three courses of stretchers to one of headers, all whole brick, until a diameter of about 3 feet is reached, and from there to the top using three courses of squared bats to one of headers. The outside of the manhole should be plastered as the wall is built, since it may be impossible to reach it afterward. The head should be set as soon as the brick-work is completed, and the opening back-filled.

If the manhole is shallow, or for any other reason the diameter is to be rapidly reduced towards the top, this is ordinarily done by making each ring of brick a little smaller than the one below, the diameter of the manhole being reduced by 1 to 4 inches with each ring. Or it may be arched (Plate IX, Fig. 2), when the back-filling around it should be thoroughly tamped to assist in taking the thrust. In the case of flush-tanks particularly a flat iron ring is sometimes built in the outside of the brick-work at the bottom of the arch as a precaution.

Flush-tanks are built in a manner similar to the above. These, except at the very top, and catch-basin inlets, are usually larger in diameter than manholes, and are built throughout of whole brick. Extra care should be taken to have all joints filled with cement and tight, and the work well bonded. After the cement in flush-tanks and catch-basins has fully set they should be given on the inside two or three washes of neat-cement grout, laid on with a whitewash or similar brush, care being taken to cover the entire surface with each coat, which should be allowed to dry before the next is applied. This will seldom fail to give a tight wall.

No water should be turned into the trench for flushing or other purposes before the cement in these appurtenances, as well as in the sewer, has set.

If masonry in either sewers or their appurtenances is laid in freezing weather special measures and precautions should be taken. The sand, stone, brick, and water should all be heated before being used, and special care taken to see that no ice or frozen dirt is in the mortar, on the stone or brick, around the sub-drain, under the pipe, or under or behind the brick or concrete sewer-invert. To insure the last it is well to take out the last foot or two of trench just before the sewer is to be laid in it. If any frozen earth is found under the

sewer grade it should be removed and replaced by sand or gravel thoroughly rammed.

The water for mortar can be conveniently heated by injecting into it steam (as the exhaust from a pump- or excavator-engine), it being kept in several hogsheads or oilbarrels. The brick and stone can be heated by piling them as in brick-kilns and burning a wood fire under them; and the sand by being piled over these, or in large iron pans such as are used for heating asphalt.

Piles are ordinarily used for sewer-foundations in soft soil. They usually support a timber platform, but in some instances concrete is placed directly upon and around their heads. For driving them the ordinary pile-drivers are used, or they are sunk by the water-jet. If they are to support platform timbers they must be driven carefully to line and sawed off accurately to grade. It will sometimes be advisable to drive the piles before the excavation has proceeded very far, using piles considerably longer than actually required, as the jarring of the banks of the trench may thus be avoided, as well as the inconvenience of moving the driver through or over a trench 'full of braces. The objection to this plan, aside from the cost of the additional length of the piles, is that they interfere with the excavation.

In moving an ordinary pile-driver through the trench it will be necessary to remove the braces ahead of it. But no brace should be removed until another has been inserted behind the driver-frame between the same rangers and as close to the first as possible. This trouble might be avoided in many cases by placing the pile-driver on a track, on a level with the ground, over the centre of the trench; or the track may be on the surface at one side of the trench. The driver

is then provided with movable hammer-guides, which can be lowered into the trench and raised with ease. The use of the steam-hammer pile-driver is often advantageous, and in sandy soils the water-jet can be used to advantage. Neither of these last is interfered with in its operation by the bracing.

The dimensions and construction of the platform follow the rules for ordinary foundations. There is usually but one set of timbers under the planking, which is in most cases composed of one or two layers of 2-inch to 4-inch plank, as in Plate VI, Figs. 3, 5, and 6; although in some instances heavy timbers are used, as in Plate VII, Fig. 10. Any timber which is to be placed where it will not be continually wet should be creosoted.

If a platform is used without piling, sills, longitudinal or cross, should be placed under the planking, although in the case of small sewers these may consist of lengths of 2-inch plank only. Platforms without piling or heavy sills are of little permanent service under large sewers, but during construction may serve to prevent local distortion of the masonry before the cement has set. One or two lines of plank placed lengthwise under a pipe sewer, however, are in many cases of permanent value, back-filling being thoroughly filled and rammed between the pipe and the plank.

Among the best of our woods for foundations are the cedar, oak, elm, alder, and beech. All bark should be removed and the sap dried out from piling or sawed timber. The platform timbers should be fastened to the piles with iron drift-bolts or treenails.

ART. 77. PUMPING AND DRAINING.

Next to quicksand, water is probably the worst enemy of the sewer-contractor and requires a large share of the attention of the engineer. If there is but a small trickle or ooze

If the

of water into the trench it may interfere but little with the excavating, and will collect at points in the bottomed trench whence it can be removed at intervals by a bucket. amount becomes somewhat greater it may still be handled without the use of sub-drains, that from where the pipe has been laid being shut off by the back-filling.

The amount from the trench ahead of the sewer may need to be pumped, however. For removing small quantities of water from a trench probably nothing is better than a diaphragm-pump. Tin "boat-pumps are often used, but will not handle so much water, are less economical of power, and are not so convenient as the diaphragm-pump; they can, however, be used in trenches more than 20 feet deep, where the diaphragm is hardly practicable. Under favorable conditions a diaphragm-pump can be made to raise 5000 or 6000 gallons per hour. Diaphragm-pumps can be used in deep trenches by placing a second pump upon a platform half-way down the trench, which discharges the water into a tub, from which the first pump raises it to the surface. Or the upper

pump may not be used, but a trough may carry the discharge from the lower one to an opening in the sewer at a point where the cement is so set as to be uninjured thereby, the water flowing through the sewer to its outlet.

A sump-hole of ample size should be made in the bottom of the trench to receive the suction-pipe, which should be provided with a strainer at the bottom. If the material is sand or soft ground it is well to place a pail or keg in the sump to keep the end of the suction-pipe from being buried, the top of the pail being just below the level of the trench bottom. The pail should be watched and material kept from running over its edge. The excavation should usually be so carried on that the whole trench slopes toward the sump-hole, each laborer seeing that the water flows through his section to the next lower.