Now, in the ease of the first objection, it does appear fea sible, that a small pipe would be more rapidly filled up than a large sewer, but yet there are some grounds for supposing the contrary to be the case ; for as the water-way becomes contracted, so also does the scour of the stream increase ; and moreover, the stream being pondcd back, we shall have a full head of water to increase the power of the flow, so that it would appear, that unless the matter causing the obstruction be extremely difficult of removal, the water-way is not likely to be entirely closed up.
The second objection must be allowed to possess consider able weight ; in case of stoppage, it would, in truth, be a matter of great difficulty and inconvenience to remedy the accident ; yet, at the same time, we must not forget, that even in brick sewers, if' they be of the smaller class, there is no very ready means of access ; and moreover, they are more likely to encourage deposit ; we are inclined to think, that if pipe-sewers be carefully laid, and fairly used, they would not fie likely to silt up. The objection to breaking up the roads, for the purpose of discovering the stoppage, can scarcely he overrated ; and it is also true, that it is very difficult to relay the pipes in their former position, unless indeed they be made with half-socket joints, or be made of two semi-cylin drical pipes one laid over the other.
The nicety required in laying the pipes, and the difficulty and uncertainty attending the practice, is certainly objection able ; there is no means of seeing the interior of the pipe, to observe how the work is performed, and it is not unreason able to suppose, that the cement at the joints, by the care lessness of the workmen, is sometimes left to form a project ing ridge above the surface of the invert. To prevent any obstructions at the joints, it has been proposed to make the pipes of a slightly conical form, inserting the smaller end of the first into the large end of the succeeding pipe; this, however, seems rather an awkward method of meeting the difficulty ; and we are inclined to think, that a better method would be, to make the pipes in two pieces, as suggested above, first laying the inverts and securing their accuracy, and then covering them over with the upper half; but neither is this plan without objections.
As regards the separation of house from surface drainage, so long as the latter bears so small a proportion to the former, we do not think it advisable to form a separate system for each ; but in eases where the proportion of surf ice-water is increased, and the house-drainage tolerably uniform, it may possibly be advisable to separate them, if the sewage be intended for dry manure ; otherwise, we should scarcely deem it expedient : with reference to the size of sewers, we do not 1 think it necessary to provide for extraordinary storms; we would rather have the advantage of a good general drainage, and the occasional and temporary inconvenience caused by a heavy storm, than be safe from the latter, with the constant inconvenience of imperfect drainage. Generally speaking, we should advise a system of pipe.sewarie,somewhat larger in pro
portion to the area drained, than that observed in those laid down for experiment in Earl-street ; for besides, that on such occasions the pipes are likely to be more carefully laid than in ordinary cases, we find thaton one occasion during that month, the water reached a height of 9 inches above the head of the pipe. We would allow flir such storms as are likely to occur once a month, but not such as occur only once in three or foul years. On this one occasion, at Earl-street, the dis charge was above 300 cubic feet per minute, and as a similar overflow is recorded in Upper George-street, we may reckon that such discharges are not unfrequent during the rainy season : we must not forget, however, that the pipes in these instances were connected with an imperfect system of colla teral drains ; and that under a perfect system, the surface water would have been carried off more rapidly, and there would not have been so great an accumulation at this point. A little experience of the working of a perfect scheme of this description, would readily determine the requisite sizes.
In main lines of sewer, mid where they pass under main roads, we should recommend the construction of brick sewers of such size as would be sufficient for the passage of a man, and for room to work in when requisite. In these we would lay not only a pipe-sewer of sufficient capacity for the usual run of water, but also the gas and water-mains, where prac ticable, in order to obviate any necessity for breaking up the roadway on occasion of repairing, &c. We are convinced that this arrangement is of considerable importance, where the roads have to sustain much traffic, for it is impossible to maintain good roads while they are constantly being broken up by gas and water companies. The expense of this system, as far as regards the sewers, would not be much greater than the present, and there can be no doubt but that the manage ment of the roadways would be much more economical. Under such circumstances, it would be worth while to con struct a good solid roadway, which, though a little more expensive in the first outlay, would in the end prove much more economical, not only as regards the repair, but also as requiring a less amount of tractive power, and doing less damage to the vehicles passing over it. It is very advisable, that all subterranean works should be treated of together as separate portions of one system. The pipe-sewers should be laid in the invert, and might be so arranged as to allow of an overflow into the large sewer, when requisite ; or the water might be poured back, for the purpose of flushing when deemed advisable. Access would be obtained into the sewer, for the purpose of inspecting and repairing the various pipes by side entrances, as at present; and the sewer ventilated by shafts in the roadways, without detriment to public health, the pines which convey the sewage being impermeable. The only diffi culty which limits the application of this system to main roads and sewers, is its expense ; were it not for this, it might be adopted in every street with much advantage.