When bricks abound as a building material, they are particularly convenient for the construction of deep sewers and drains, from the facility of handling in confined spaces; but it is important their quality should be of the best, since if they scale and decay, great expense must be involved in the repair of the drain. The Tipton, or blue brick, is the best for the facework of drains.
In parts of the country where stone abounds, bricks are often little known, and the resources of the district must be made use of; where the blue lias limestone occurs, I have found it a cheap and excellent material for forming culverts and drains of all sizes; and it was used largely for that purpose on the Birmingham and Gloucester Railway.
Annexed is a sketch of the sections of drains varying in form according to their size. The batter which I found most useful and convenient for wall-sided drains was 1 in 9, either curved or straight; the first is the best form in theory, but in small works I found the bricklayers’ and masons’ work more accurately to the straight batter; and the last is, from its simplicity, better adapted to receive any sluices or flush-gates that may be necessary.
Annexed is also a sketch to show the distribution of drains in a town supposed to be built on a regular plan, with a pretty uniform descent towards an axis, which constitutes the site of the main drain; each class of drains consisting of several sizes, it would be most useful as well as economical that the drain of a particular class (if large) should commence with the smaller size, and discharge or terminate with the greater size, a plan that would aid the sewerage of the water.
In a system of drainage, it is necessary to consider that the greater the body of water, or in other words, the class of the drain, the less declivity is sufficient; and the converse, the less the body of water, or class of drain, the greater declivity is required; in the first case, the hydraulic depth compensates for the want of declivity; and in the second case, the declivity compensates for the want of hydraulic depth; the multiplication of these qualities being a function of the velocity or force of the current, due attention to the above is important in economizing or turning to the best account the declivity for the drainage of a large town.
Having arranged the system of sewerage for a town, the next object will be to render it as extensively useful as practice will admit of; and from the experiments and practice of Mr. Roe, the surveyor to the Holborn and Finsbury Commission of Sewers, we are warranted in the belief that a good system of sewerage, aided by a sufficient supply of water, will, in most localities, be sufficient to remove all the dirt which arises in the streets, without the necessity of cartage, and also all the filth of private dwellings which is at present led through drains or pipes, or which by the aid of water may be practised more extensively in future.
We have first to consider the conveyance or discharge of the street dirt into the main sewers, and the discontinuance of the present expense, and annoyance of using carts for that purpose, at least with some few exceptions.
It is pretty obvious, that if the mud of London, like water, could be made to flow through the drains much trouble and expense of cartage would be saved; and it does happen that the street-dirt of London is so diffusible with water, that with a little arrangement such a mode of cleansing maybe followed; indeed it is highly probable that at present more than one-half of the whole mud is carried off by the rains in that manner.
The mud of London, and other great towns in England, may be assumed in wet weather[[51]] to arise, in three-fourths of its amount, from the grinding or abrasion of the paving-stones, the remaining one-fourth part consisting of soot, shop-sweepings, and cattle dung.
The dirt arising from the detritus of the stones may be obviated in two ways; 1st, by substituting for the green-stone forming the carriage-way, quartz-rock, or quartzose stones. The green-stones contain hornblende and felspar, which grind, like all argillaceous stones, into fine mud or powder mixable in water, whereas quartz rock retains when ground the form of clean sand, neither soiling nor capable of forming mud in itself. The Lickey Hills in Worcestershire are composed of quartz rock, and the roads in their vicinity show its excellence as a material for road making. The quartz rock, however, of the island of Jura is much purer, and that island contains an inexhaustible supply already broken by nature into sizes nearly fit for laying on the roads; and Small’s Bay in the island of Jura would form a convenient loading place, and by means of a jetty and tram-way vessels might be laden at a small expense, and much of the country supplied with the best of all materials for road-making. The substance of the stone is hard and durable, and consequently suffering little by abrasion; and it would be well worth while to try the experiment of Macadamizing one of the leading streets of London with this material, as the means of forming a good road, and at the same avoiding the creating of a great quantity of street dirt.