Page:Architectural Review and American Builders' Journal, Volume 1, 1869.djvu/814

 656 The Architectural Review and American Builders' Journal. [April, water is fearful, and some streets are perfectly inundated during, or immedi- ately succeeding a rain-storm. The drainage from houses should not be a superficial operation, crossing the sidewalks, but should be carried under them, and have a sufficient fall to the curbstones to prevent any lodgement of water and consequent freezing. These cross drains should have sufficient capacit}', and be made smooth, to pre- vent the lodgement of any obstruction to the free passage of rain-water from the spout, with which it should be con- nected in the best manner. But all that we have said relates to surface-draining only. We will now descend to the still more important work termed sewerage, into which drainage may be said to merge itself. In this age of sub-cellars descending, on an average, twenty feet below the surface of the street, it would be very desirable, indeed, to have the main sewer built low enough to receive the drainage of such cellars ; and thus avoid that pumping which is so great an ex- pense, in many cases, in our large cities. . We need not, of course, offer any counsel to the judicious experience of the engineer entrusted with the very serious commission of surveying and mapping the main sewer of a village or city, either already commenced, or in prospect. But, if he has the initiatory work to do, he must certainly possess a very great advantage over the profes- sional brother who has to deal with streets already built, and is confined to levels which he must pursue. The egg-shape is decidedly the best form for a sewer, for it has not alone the great advantage of strength, but .that, also, of narrowing the invert, therebj' contracting the lower part of the channel, and increasing the depth of flow, by which means the velocity and scour are much improved. Another advantage of this form, is the attain- ment of greater height with a given area ; and, in fine, the construction is more economical. Various methods of constructing sewers approximate to this egg-shape. Some have been made with a semi- circular top, flat segment sides, and a sharp, segmental invert ; the curves of those sides and invert, differing accord- ing to the varying proportions of height and width, and other circumstances. Others are constructed with six centres, the upper part being described with two radii, instead of being semicircular. The most to be preferred of all the modes and proportions is the following : Let the height of the sewer be to the width as three to two ; then, describe a semicircular crown ; and with a radius equal to the width, or span, taking each of the springing points of the semicircu- lar top for the respective centres, de- scribe segments ; for the invert, with a radius of either one-half or one-sixth of the whole height, describe a semicircle which will connect with the foregoing segments, and thus complete the con- struction. It must be remarked of this egg- shaped sewer, that the form is inconve- nient to walk up,- when workmen have to examine it, or make repairs. A flat bottom would, unquestionably, be the most fitting for that purpose ; and many scientific and practical men are of the opinion that the latter shape would be the best for perfect drainage. As much might be advanced in the way of argu- ment on both sides, we propose to leave the question open. Should it happen that the grade of a road or street might be so low as to prevent the height being such as we would advise, two, or even more, egg- shaped sewers might be laid to make up the aggregate area required. The above sewers, if not more than four feet in height, and to be constructed in a good building strata, may be of half-brick work, on account of the in- trinsic strength of their form. But, if built according to the other modes, can-