The bottoms of manholes should always be formed of concrete, pioperif prepared and laid to the shape of the invert, which should be accurately moulded to receive the channel. This channel should be formed of highly-glazed ware, and may lie either socketed or butt-jointed There are various ways of finishing the remainder of the bottom of the manhole; sometimes cement rendering iused, others prefer salt-glazed or blue Staffordshire bricks, and recently manhole bottoms have been made entirely in one piece of highly-glazed fire-clay. This forms an excellent and clean l>ottom. and is known as Borders pattern; it is shown in Fig. 341.

Fig. 341   Border's Glazed Fire   clay Manhole bottom.

Fig. 341 - Border's Glazed Fire - clay Manhole-bottom.

Fig. 342   Curvred Glazed Fire clay Channels for Manhole.

Fig. 342 - Curvred Glazed Fire-clay Channels for Manhole.

Drains must be laid in perfectly-straight lines from wall to wall of manhole, all bends being made in the manholes. Suitably-radiated channels, such as are shown in Fig. 342, are made for this purpose. Branch-drains should, wherever practicable, be connected in the manhole, and should be joined to the invert-channel by properly curved pieces. These are made in a variety of forms. The channel-bends of Mr. Winser (Fig. 343) have an overhang on the outer side to prevent the escape of sewage on to the floor of the manhole, and the branches are higher than the central channel. "Wyvurst" channel-bends, with loose domed pieces, as shown in Fig. 344, are also very useful.

A change of level in a drain can be most conveniently effected at the manhole It is very objectionable to connect the drain from the higher level directly with the manhole, as the splashing leaves portions of the Bolide on the floor and sides, and this of course renders it impossible to utilize the manhole for inspection purpose The proper way is to place on the drain, immediately behind the manhole wall, a right-angled junction with the junction downwards, and in the same wall at the invert level to place a bend, and to connect this bend and junction by means of a pipe-shaft, securely surrounded with concrete.

Fig. 343   Winser's Channel bends.

Fig. 343 - Winser's Channel-bends.

For inspection purposes, the pipe beyond the right-angled junction can be built into the wall of the manhole, and fitted with a stopper, which can be removed whenever inspection is required. A very secure clip for holding the stopper in position is made by Doulton & Co., and is shown in Fig. 345. This method of bringing about the change of level in drains is shown by dotted lines in the longitudinal section of the manhole in Fig. 339.

Fig. 344.   Wyvurst Channel bends.

Fig. 344. - Wyvurst Channel-bends.

The walls of manholes should In- built of brickwork, and as there is necessarily a great deal of moisture in manholes, the facing-hricks should be as hard and impervious as possible. In many districts the ordinary common brick is not suitable for use in being; some people therefore run to the other extreme, and use best white-glazed bricks. This is an extravagance, as they possess DO solid advantages over the much cheaper wire-cut brindle, or common brown salt-glazed bricks. The mortar should be made from Portland cement or hydraulic lime, and all the bricks should be bedded as closely as possible, no joint to exceed 1/4 inch in thickness. All arches should be formed of purpose-made bricks of the proper radius.

For the purpose of access to the drain from the surface, step-irons should be built in the brick-work at suitable intervals, say, every fourth course. Steps made from cast-iron are not to be commended, as they are frequently broken by the accidental dropping of heavy objects when the cover is open. Wrought-iron steps are the most serviceable. These should be of inch round wrought-iron, formed to project about 4 inches into the manhole, and about 9 indies wide, and should be well coated with hot tar as soon as they are made.

The best stone for manhole-covers and landings is that known as York paving, which is obtained from the district of Brighouse in Yorkshire: a similar kind of stone is also obtained from the other side of the hills in the Rossendale district of Lancashire. Where the manholes are in such a situation as likely to be traversed by heavy loads, the covers should be of "landings" 6 inches thick; in other situations they should be selected from 4-inch flags. They should be holed at least 20 inches in diameter, or of other sizes to coincide with the opening of the entrance cover.

The cast-iron manhole-covers are made in an endless variety of patterns, and may be divided into two classes, one being the open ventilating cover, and the other the air-tight cover, both being cast in heavy and light patterns. If the open cover is fixed in a situation likely to be traversed by heavy traffic, such as coal-carts, Ac, a strong pattern should be used, and fitted with a galvanizediron dirt-box; the light patterns can only be used in situations entirely free from traffic. The closed covers may be used where air-inlets are not required, a light-hinged cover being suitable for the purpose, except close to buildings, when the chambers should always be protected by what are known as "air-tight covers" (figs. 346 and 347). These are generally made by having a strip of india-rubber in the frame, into which the cover presses when closed. Instead of india-rubber, a groove in the frame is sometimes filled with sand or with oil, glycerine, or soft soap, and in addition the cover is screwed down into the frame. Where extra precautions are necessary, a "double-seal cover" may be used, as shown in Fig. 348.

Fig 343   Doulton's Clip for Holding Stopper in End of Drain.

Fig 343 - Doulton's Clip for Holding Stopper in End of Drain.

Fig 346   Addam's Air tight Manhole cover (light Pattern)

Fig 346 - Addam's Air tight Manhole-cover (light Pattern).

It is desirable to have access to all drains about every 80 yards but while manholes should be built at every change of direction and gradient, it is not necessary to place them upon long straight lines, except alternately with lampholes. The lamp-hole 1- simply a plain pipe shaft, carried up to the surface, but to make a satisfactory piece of work, this shaft should be surrounded with a collar of concrete, so as to ensure a solid bearing for the stone cover, upon which the cast-iron ventilating cover is to rest When the drainare shallow, great expense may l>e saved, if, instead of constructing manholes at the turn-.

Fig. 347   Adam's Air tight Manhole cover to receive Concrete Filling.

Fig. 347 - Adam's Air-tight Manhole-cover to receive Concrete Filling.

Fig. 348  Section of Jones's Airtight Douhle seal Manhole cover.

Fig. 348 -Section of Jones's Airtight Douhle-seal Manhole cover.

Fig. 349   Plans and Section of Tron Inspection  eye for Bend.

Fig. 349 - Plans and Section of "Tron"Inspection -eye for Bend.

1 A somewhat similar cover is made to receive a stone slab.

the "Tron" inspection-eyes are used Fig.

349 shows plans and section of one of these bends; the circular socketed eye-piece may be either 15 inches or 18 inches in diameter. When required to be brought to the surface, a straight length of pipe is jointed on, and the lamp-hole cover fixed in the usual way. Not only are these eye-pieces made to various curves, hut they are also made with right and left oblique junction-, double-junctions,

T-junctions, and Y-pieces. A four-way junction with eye-piece is shown in

Fig. 350. They can also be fitted with Stanford's or any other special form of joint if require.

Fig 350   Plan of Tron Inspection  eye Four wayjunction.

Fig 350 - Plan of "Tron" Inspection -eye Four-wayjunction.