DISCUSSION. AN ACCOUNT OF THE DRAINAGE OF RICHMOND, SURREY, UNDER THE AUTHORITY OF THE METROPOLITAN COMMISSIONERS OF SEWERS, IN 1851.

With'regard to the flushing of sewers, it ought to be borne in mind, that the heavier matters held in suspension in the sewage, or driven along the sewers by the force of its current, never moves with the same velocity as the water, and consequently, with intermitting currents as in house drains, it is often left dry and sometimes becomes indurated, so that it cannot be moved by the succeeding flow of water ; hence the necessity of a continuous current of water in all pipe sewers, to prevent the accumulation of matter within them.The sizes of all house drains and pipe sewers should he such, that they should never be more than four-fifths filled with water, or sewage, so as to allow of a free circulation of air through them, at all times, as the air not only facilitates the flowing of the water and promotes ventilation, but it also prevents the accumulation of highly-concentrated gases and their rising through the drains into the houses.T h e paper is illustrated by a plan of the sewers of Richmond.Mr. DONALDSON said, that at present, the drainage wm conducted into the Thames, the main end outfall sewers not being yet completed, brit it was intended to connect them, ultimately, with the Kennington sewer.The largest pipes he had employed, were 20 inches in diameter, but pipes of that size were rarely made so perfect as those of smaller dimensions, and there was great diEculty in well securing the joints.Where a larger diameter than l 8 incheu was required, he should prefer, on the score of safety, to construct a brick culvert, of which the cost would be about the same: the Contractors, indeed, finding at the time, some difficulty in obtaining the 20-inch pipes, offered to construct instead, a 30-inch sewer of bricks in cement.If pipe sewers could be made large enough to carry off the surface waters, and strong enongh to bear the pressure of the superincumbent earth at great depths, he should prefer then) to brick drains, provided the cost were the same ; for the damage to drains arose not only from the wear and tear, but also from the rats, who could not attack the pipes, which should be carefully jointed with clay, not with cement.The use of pipes was, homever, subject to this inconvenience, that whenever it became necessary to make a new junction, it was difficult to avoid breaking them in opening the ground, and thus a greater expense was often incurred, than would have resulted from the use of brick drains.Mr. HERTSLET said, the most striking feature of the plan was the employment of the separate system.There was no doubt, that