other permanent masonry structures, bench-marks may be cut or otherwise indelibly marked on their surface, for the guidance of the conservancy staff. It will then be comparatively easy to maintain the level, when the drains are being excavated and silted. A convenient form of level is of importance in making or excavating drains, in fact where there is little fall, the use of a reliable instrument is quite requisite to grade the bottom of the drain at a true inclination, so that the sewage water may flow steadily. The ground surface is often so deceptive that levelling by the eye and random guessings should never be trusted to. The figure (A) as under, represents a

simple, cheap, but efficient drain-level that any carpenter can make in wood in an hour's time; A A are pieces of wood one inch thick, eight feet long, four inches wide at the lower ends and two inches wide at the top; B is a graduated cross-bar screwed to A A; P consists of a plummet and line. Before marking the graduated scale on B, let the level be turned halfway round, if the plumb-line indicates the same point or mark, the level is practically correct. The pieces should be planed smooth, painted, and the joints screwed together. Templates are useful for forming the banks or slopes: they

Mischief of Irregular Excavation.

107

can easily be constructed as shown in figure (B) of any

straight wood, three inches wide, and one inch thick.

Unless the bottom of a ditch or drain is made with an uniform gradient, the earth being soft and readily washed out in some places, and more compact in others, the bottom of the channel would be liable to be gullied where the water runs most rapidly.

No excavation or silting out must be permitted below the level of the bench-marks.

Where a long course of unscientific silting out has lowered the bed of a drain beyond the proper level, it should be raised with sound earth, clay or building rubbish, well rammed and beaten down so as to form a smooth impervious bed. As a general rule, it will be found that the remedy required consists in raising, instead of deepening, the beds of all old cutcha drains.

In raising the bed-level of drains, the work should always be commenced at the highest point of the drainage. Whereas in excavating or silting, the coolies should invariably start from the point nearest the outfall, and work up the drain. Wide mischief is caused by careless, aimless, and irregular excavation of drains at their upper portions, thus often necessitating an undue deepening of

108

Retaining Walls and Drain Bridges.

the outfall end if the water is to be allowed to flow off. An ignorant overseer or road jemadar thinks he has done a praiseworthy work if he has carefully cut out the bottom and sides of the cutcha branch drains in his immediate section, leaving a dry clean bottom and well-trimmed slopes, without giving a thought to the consequences of lowering the bed of the drain at that point, possibly lower than the outfall itself.

All culverts, bridges, and covered ways should be constructed of the full width of the waterway of the drain, and with masonry floors or inverts placed a couple of inches lower than the bed of the drain to permit of proper silting without disturbing the level. Retaining walls and other structures placed alongside the drains must have their foundations sufficiently deep not to be disturbed or endangered by ordinary excavation or silting. All bench-marks, inverts, floors, arches, and other masonry structures should be built of sound, hard, wellsoaked bricks, laid with first class hydraulic lime mortar and the joints raked and pointed with cement, composed of one part fresh Portland cement and two parts clean sharp river sand. What is called cutcha-pucka masonry should never be tolerated: it has no stability, and the bricks being porous, and the mortar mud, or very inferior soorkee and lime, the open soft joints soak in the sewage, become intensely foul, and more difficult to cleanse than a simple clay bottom or bank.

Rough piling of the drain sides with wooden or bamboo pins, and bamboo, stick, or rough plank platforms, should never be allowed. The extension of shop frontages into the street is a fraud upon the public. Where bridges are required for access to shops or stables, or

110

Sewage Flow in Drains.

they tend to reduce the width of the streets and are unwarrantable encroachments on public land, being one of the means by which house frontages are gradually pushed out into the street-first comes the platform, then perhaps a temporary sunshade is placed over it, and if that escapes notice, it is replaced at some future time by a more permanent structure, till after the lapse of a few years, the fact of the original encroachment has been forgotten and the wily Hindu has "enlarged the borders of his tent" and acquired a strip of valuable frontage at no cost to himself and to the detriment of public rights. The Municipal or other local authorities who knowingly permit such encroachments, clearly fail in their duty as trustees for the public.

In forming or improving drains three questions have to be considered: the amount of velocity of the sewage flow required to carry off impurities; the capacity required to carry off the maximum quantity of water falling into the drain in a definite period; and the scouring action on the bottom and sides of the channel.

The velocity depends on the hydraulic mean depth of the stream and the inclination of its bed. The hydraulic mean depth is ascertained by dividing the cross sectional area of the stream by the width of its cross section measured along its bed. Or in other words

'By dividing the sectional area of the channel by the wetted perimeter, or the contour of the wetted channel, we get what is called the mean hydraulic depth,' or often, 'the mean radius.'”—Baldwin Latham.

Then the inclination remaining the same, the greater the mean depth, the greater will be its velocity.

The sewage flow in cutcha drains is, however, great