TABLE OF COMPARATIVE TIME OF RUN OF WATER THROUGH BRICK DRAINS AND GLAZED PIPES.
The rate of inclination is a matter which requires some consideration in laying out a system of drainage ; the greater the fall, of course the greater the velocity, and, in conse quence, the more rapid the discharge ; the scour is also pro portionately greater. It will thus be see, that the efficiency of sewers depends a great deal upon their rate of fall towards the outlet ; this, however, will depend mainly upon the nature of the locality to be drained. Supposing there exists a cer tain fall from the highest point to be drained to the outlet, it will be well to see how it can be laid out or expended to the greatest advantage :—the rule is this, give the greatest rate to the house-drains ; the next greatest, to the pipes with which they are immediately connected ; and so on, diminishing the rate gradually as you get towards the outlet. The reason ihr this is obvious ; for, in the first place. your main object is to remove the sewers from immediate proximity to the houses, so that perfect drainage is of less consequence at a distance than it is in the houses themselves. But beyond this, there is another more important object in this arrangement, for as the body of water is less in the sewers or drains more remote from the outlet, it is more likely to be sluggish in its move ments than where there is a large volume of water, and this tendency to sluggishness is overcome by an increased rate of inclination, so that the want of velocity caused by the small body of water in the smaller sewers is compensated by the extra fall, and thus the flow in all cases is rendered equal, or nearly so. In the larger sewers, where the water is collected from the tributaries, the mass of water is sufficient to pre serve a good velocity with even a small inclination. The rate will depend, as we said before, upon the nature of the ground; but, as a general rule, it is advisable that the fall in main pipes should not be less than 1 in 240, and in main-sewers not below 1 in 1,000. It has been deduced from experiments,
that no proportionate advantage is gained by a NI of more than 1 in GO. The requisite size of the pipes is dependent, in a great measure, upon their fall, for as the velocity is in creased in proportion thereto, so is also the rate of discharge, and therefore the greater the fall, the less will be the sec tional area required for the sewers.
Where one sewer discharges its contents into another, the junction should be effected by a curve, drawn tangent to the directions of both sewers, so that the direction of the stream may be changed in as easy a manner as possible, and may nut experience any shock either in leaving the smaller or enter ing the larger sewer. It was formerly the practice to make the junctions rectangular, but by this method the stream from the tributaries, crossing that of the main at right angles, had a tendency to change its direction, the amount of change depending on the comparative force of the two streams ; in any case, however, the flow in the main stream was impeded, eddies caused, and deposit thereby accumulated. lf, on the contrary, the two streams be tangential at the point of junc tion, little or no impediment will take place, as they have both a tendency to move in the same direction. It is advi sable, that the curve be struck with as large a radius as practicable. In the pipe-sewers, junction-pipes are made for the purpose, in which the junction-curve is commenced on the main-pipe, so that we have a straight pipe, and a portion of a curved one, in the same length; the pipes are made to suit different circumstances, some with one, others with two junctions, and these of various sizes as required. With respect to the time occupied by the passage of water through different junctions, the following results have been arrived at by Mr. Roe, after various experiments:— Time occupied by the passage of equal quantities of water through similar lengths, and with the same inclination. Along a straight line-90 seconds.