reinforced. The reinforcement, however, tends to prevent rupture if cracking occurs; and with flat slab roof or side-wall construction, or flat inverts on yielding soil, reinforcement is necessary unless excessive thickness of concrete be used. In concrete arches, it is considered good practice to use transverse reinforcing bars to the extent of 0.2 per cent to 1.5 per cent as much cross-section of steel as of concrete, depending upon the factor of safety desired against unforeseen stresses or poor workmanship. In any concrete sewer there is likely to be contraction and expansion due to temperature changes. The former is apt to form transverse cracks. If longitudinal reinforcement be used, it tends to prevent wide cracks, distributing the contraction into a great number of cracks too fine to cause leaks. Reinforcement to the extent of about 0.2 per cent to 0.4 per cent of the concrete area is common practice. In some sewers, in place of or supplementary to reinforcement, expansion joints are used in the concrete at intervals of 25 to 50 feet, generally of the tongue-andgroove type. Even if these are provided, it is best to use about 0.2 per cent of reinforcement, especially if the joints are more than 25 feet apart. ART. 42. MANHOLES, FLUSH-TANKS, AND INLETS The purpose of manholes, as the name implies, is to give admittance to the sewers, which is necessary for the purpose of inspection and cleaning. They should therefore be sufficiently large to permit a man of average size to enter and work in them. They are also utilized for ventilation. Manholes are in general built immediately above a sewer and leading from it to the ground surface. In the case of some large sewers in Europe, they are built at one side of the sewer and connected with it by an underground passage, the chief advantages of which construction are the greater convenience for entering and the avoiding of manhole-heads in the street-pavement. But this construction is very expensive, and the passage is liable to be a collector of filth. The size of vertical manholes is usually 24 inches diameter at the top, although sometimes only 22 or even 20 inches, increasing towards the bottom to a size in which a man can work. The least size advisable for the bottom on lines of pipe sewers is 4 feet circular or 3 feet by 4 feet 6 inches oval. In manholes of this size, the ordinary operations of inspection and cleaning of pipe sewers can be carried on. There is no particular advantage in having an ordinary manhole of more than 5 feet interior diameter. Manholes oval at the bottom are well adapted to locations where there are no intersecting sewers; those circular, to points of intersection. X Round Wherever possible, the sides of the manhole should be built vertical from the side benches of the bottom, ab, cd, Fig. 39, to a point 3 feet above, from which point they may be brought in with a batter or corbells to the smaller top, which is usually circular. Where the depth of the top of a pipe sewer below the surface is less than 7 feet, this construction becomes difficult, owing to the considerable batter that must be given to the upper walls, since the slope cannot well begin at a lower point than that stated and leave working room at the bottom. If the depth of sewer is more than 5 feet, this difficulty can be met by arching the walls (see Fig. 40), which construction requires careful workmanship. FIG. 41.-MANHOLE An alternative method, especially adapted to a depth of less than 6 feet, is to reduce the area of the manhole near the top by an offset, using either a brick arch or an iron beam to span the offset (see Fig. 40). If the manhole is more than 10 feet deep, the diameter should increase more rapidly for the first 3 feet down from the top, being at least 2 feet 9 inches at that depth, as otherwise descent through the shaft will be difficult. Descent through the manhole can be made by means of a ladder, but it is customary to build steps into the wall for this SECTION ON A-B Iron Trough -B FIG. 42.-CROSSING MANHOLE. purpose. These may consist of protruding bricks or stones, but preferably and commonly are of cast or wrought iron. Iron steps are made of various shapes. The simplest and probably as good as any is one made of a round wrought-iron bar bent and the ends flattened as shown in Fig. 41. The steps should be placed about 14 inches (6 bricks) apart vertically, and either directly under each other or alternating on opposite sides of a vertical line, the former in narrow shafts. Where one sewer crosses another without intersecting it, a manhole of special construction, permitting of inspecting each sewer, is desirable. Such a one is shown in Fig. 42, in which the upper sewer is continued through the manhole by an iron trough. While, at the junction of a pipe-sewer main and lateral, the latter should be at a somewhat higher elevation than the former, it is desirable that the difference in elevation of the crowns of the Brick two do not exceed 6 inches. To not be made through the shaft of the manhole, but through a small, smooth channel. A good design is shown in Fig. 43. When sub-drains are laid under large sewers, arrangements for cleaning them may be made as shown in Fig. 44, by a vertical branch opening into a manhole; or, if they are under the center of the sewer, such a pipe may open into the sewer-invert, the |