It may be mentioned as another instance of the absence of appropriate knowledge that has governed these structural arrangements, that a large proportion of the most expensive sewers are constructed with flat bottoms. In proportion as the water is spread the flow is impeded, and the deposit of matter it may hold in suspension increased. Mr. Roe, a civil engineer, who, much to the honour of the Holborn and Finsbury district of sewers, has been appointed to the care of their sewers, and is perhaps the only officer having the experience and qualifications of a civil engineer, states, that as compared with sewers or drains with bottoms of a semicircular form, those with flat bottoms invariably occasion a larger amount of deposit; and with the same flow of water, the difference of construction occasions a difference of more than one-half in the deposit which is left. By the common and most expensive form, the drains are apt to be choked up with noxious accumulations; by being built with flat sides (instead of with curved sides, which give the strength of an arch) they are apt in clayey and slippery ground to be forced in. The expense of the improved form is nearly one-fourth less than those in general use. _Mr. Roe_, whose evidence, which is corroborated by the evidence of other engineers, is given in the Appendix, was asked,— In respect to the levels, how have you found the sewers?—They appear to have been entirely constructed with reference to the locality, to drain to the nearest outlet, and not on an extended view for the whole district, or with any view to sewerage on a large scale. In the Holborn and Finsbury divisions the Commissioners now adopt a series of levels suited from the lowest outlets to the surrounding districts. Have you heard of any alterations made in the surrounding districts on the same principle?—I have heard of none as adopted generally. The City have lowered several of their outlets; and the chairman of the Westminster Commission has had the subject under consideration for some time. What are the chief effects of the piecemeal town drainage without reference to extended levels?—Chiefly that when new lines of houses are built and require new sewers, either the old sewers must be taken up and re-constructed at a great expense, to adjust them to a new and effective sewerage, or the new sewers, if they are adjusted to the old ones, are deficient in fall, and they have greater deposits.