having disposed of these general matters, we now come to consider the subject more in detail ; but befbre doing so, let us clearly understand, and determine what we have to do, and what class of subjects we have to deal with. Our gene ral object is to convey to a distance the refuse, or a certain portion of the refuse, of the town, or other area to be drained, and what does this consist of; it is mainly of two descrip tions, viz., that which may be particularized as house-drain age, consisting of the excrements of the inhabitants, and all refuse matters connected with household economy ; and that which is distinguished as surface-drainage, which has espe cial reference to rainfall and storm-waters. Formerly it was the custom to make no distinction between these two descrip tions; but of late, we have begun to consider them as sepa rate items, and a grand question has been raised as to whether it is correct to provide for them both by the same system of drains. In the one case, the supply is, or might be, made tolerably uniform ; in the other, it is periodical and uncertain, nor can it be made otherwise; it therefore becomes a ques tion, whether the two can be advantageously combined, but as the inquiry is so mixed up with the determination of the size and shape of sewers, it will be better to consider it under that head. We have made no allusion to subsoil drainage, for it is indeed very rarely required ; if the surface be pro perly drained, there will be no necessity for subsoil drainage, except in cases where springs exist, or where the strata con tain water, and, even in such cases, it is questionable whether the water might not be usefully employed, rather than carried away at a considerable expense. We shall leave this part of the subject therefore out of the question, and turn at once to the consideration of the sizes and forms of sewers.
Until the last few years, it was the custom to construct sewers of a very large size, varying in sectional area from 5 to 100 feet, the former being allotted to collateral sewers in courts, alleys, and side-streets, and the latter to the main outlets into the Thames, the average area for main lines being about 12 feet. Now, the fact is, that these sewers were seldom, if ever, fully charged, the sectional area of the stream usually flowing through them, averaging not more than is part of the area of the sewer, and the flow being often scarcely perceptible. In continuous wet weather, the area of sewage was greatly increased ; and also on such days as fresh water was supplied to the houses; in neither case, how ever, was the sectional area equal to that of the sewer, and the only occasion on which the sewers were anything like full, was the occurrence of some very heavy storm, such perhaps as does not appear once in the course of a twelve month. Now, it appeared to those who have recently brought forward this subject tier investigation, that a much smaller size would answer the purpose much better, and be decidedly much more economical in construction. Under this impres sion, several experiments were made under the Metropolitan Sewage Commission, to test the filet, and the result appears to have been decidedly favourable to the new theory.
In a main line of sewer in Upper George-street, Edge ware-road, which communicates directly with the King's Scholars' pond sewer, and which is 5 feet 6 inches in height, by 3 feet 6 inches wide, an earthenware pipe 12 inches in diameter was inserted, laid immediately upon the invert of the larger sewer, and a head-wall built up above it in the sewer, for the purpose of preventing any sewage-matter passing out by a different channel. The total length of this pipe measured 500 feet, and the whole area drained through it amounted to about 44 acres. The results are stated to have been as follows :—" The velocity of the stream in the pipe has been observed to be 41 times greater than the velocity of the swine amount of water on the bed of the old sewer. The pipe has not been found to contain any deposit, but during heavy rains the stones have been heard distinctly rattling through the pipe. When the pipe is nearly the velocity and concentration of the water are sufficient to clear away any matter which may have been drawn into the pipe from the large sewers, much of which matter, it may be presumed, would never enter a well-regulated system of pipe sewers; also the force of the water issuing from the end of the pipe, is sufficiently great to keep the bottom of the old sewer perfectly clean for 12 feet in length; beyond this dis tance, a few bricks and stones are deposited, which increase in quantity as the distance from the pipe increases. Beyond a certain distance, mud, sand, and other deposits, occur to the depth of several inches, so that the stream there is wide and comparatively sluggish, and being dammed by the deposit, exerts an unfavourable influence on the flow of water through the pipe. On the invert of the original sewer, which now forms the bed of the pipe, deposit was constantly accumu lating, and was only partially kept under by repeated flushes. The superficial velocity of the water in the pipe is generally three, four, and five times greater than the superficial velo city which obtained, under the same circumstances, in the original sewer, and the velocity of the whole mass of \rater in the pipe approximates much more to its surface-velocity, as ascertained by a float, than does the velocity of the whole mass of water in the sewer approximate to its own surface velocity." Several experiments in this pipe were also made, to deter mine the propulsive force exerted by the water in removing obstacles, and it was found that quantities of sand, mud, and pieces of brick and stones, were carried from the inlet through the entire length of the pipe, where they were discharged with considerable force, and carried some distance along the invert of the original sewer. All these experiments were effected when the pipe was only or less than halffull of water. The period of year at which the experiments were made, was principally during the month of October, and the following is the detailed daily account of the depth of water in the pipe : September 2S and 29. Very wet both days and nights; there was at this-period 96 hours' continuance of rain, and the pipe was never observed to be more than half-filled.