With true curve-100 seconds.
With turn at right angles-140 seconds.
It is to be observed with reference to junctions, that it is not necessary to increase the capacity of the main line at every junction with a tributary, or that the area of the main should be equal to the area of all the junctions. The reason of this apparent inconsistency is, that although a pipe be full i at its head, it will be found to be not nearly full at its out let, and this is caused by the increased velocity of the particles of water acquired by running, down an inclined plane. At a little distance from the head, therefore, some portion of the sectional area of the pipe will be vacant, sufficient to aflord room for the discharge of a tributary, which will in its turn add to the velocity of the main stream, and leave space for a second tributary at a joint lower down. There is, of course, a limit to the addition of tributaries, for in tune the areas of the main will be comparatively full, and will dam up the water in the junctions; the extent to which the prin ciple can be carried, is to be determined by practice ; it will depend to some extent on the fall of the main line.
Where sewers are constructed with the intention of being entered for inspection, it is necessary that they should be properly ventilated; otherwise, the noxious gases generated by the sewage, would forbid entrance, except at considerable risk of life: fatal accidents have occurred, for want of such precaution. The usual plan for effecting this, is to construct a long, narrow, and tapering shaft from the crown of the sewer to the roadway ; but this practice is objectionable, on account of the gases vitiating the atmosphere, and thus encouraging disease ; and it does appear somewhat inconsis tent, that while gullies are trapped to prevent the ascent of the effluvia, other shafts are constructed for the purpose of effecting it. It is true, there is some advantage gained in ventilating by the vertical shaft, but not sufficient, it may be imagined, to counterbalance the inconsistency. Several methods have been attempted, to obviate this difficulty : amongst which may be mentioned, the connection of the ventilator with the rain-water pipes of the houses, and ex tending them a considerable height above the houses; the passage of the air through a furnace; the ventilation by steam-jet, &c.; each of which plans have been more or less
successful. If we adopt the pipe-sewers, all difficulty on this score is at once obviated, for the pipes themselves are im permeable, and if the gullies and house-drains be properly trapped with syphons or flaps, there will be little danger of the escape of noxious vapours; and it must be remembered, that hf this case, the sewers themselves are not required to be ventilated, seeing that they do not admit of passage by a man. That they are ventilated, however, to a certain extent, is very probable ; for it is well known, that the rapid motion of a stream of water will produce a sensible current of air, and in proportion as the velocity of flow is greater in small than in large sewers, so will their ventilation be more com pletely effected : besides this, as the deposit is likely to be less, there will be less need of ventilation.
In large sewers, it is necessary that there should be ready means of access to them, and this is best provided for by side entrances, which consist of vertical shafts descended by iron ladders or step-irons, and covered with cast-iron flaps. These shafts are placed at any convenient spot, at the side of the sewer, generally in the footpath, where the sewer is in the middle of the road, and from the bottom of the shaft an arched passage is carried with a slight inclination under the roadway into the sewer, the lowest level of the bottom being from G to 18 inches above the invert.
Gullies require to be constructed at certain intervals, to carry of the surface-water from the roads. The water is carried by gratings in the channel of the road into a cesspool or well, and thence by an overflow-pipe with syphon trap into the sewer. The grating should be made to open, or other access afforded to the cesspool, in order to remove the deposit when requisite.