It has been suggested with reference to the drainage of London by the Thames, that as the flow is of only 5 hours' duration, and the ebb of 7, and that during that time the range of flux is 7 miles, and reflux 10, therefore a pregres sive movement of 3 miles is made towards the outlet every ebb and flow. This conclusion has been arrived at without sufficient data, forait has since been established by actual experiment, that not only is there a progressive and retro grade motion going on every tide, but that the same process takes place with a series of tides, and that after a float has made a certain progress down the river by the excess of the flux °vet the reflux, it turns, and makes an actual retrogression up the river, by the excess of the reflux over the flux, until it arrives once more nearly up to the point whence it started.
From this it will appear that, in a tidal river, there \rill be a great difficulty in carrying refuse matters to sea, or even from the vicinity of towns, if it be allowed to discharge at or near the town ; it will constantly be carried about back wards and forwards, and depositing its solid matters in the bed, or on the banks, of the river. The sole remedy for this seems to be, to extend the sewer to such a distance from the town for discharge, that the tide may not at any time be able to carry it back near the town. Another object will be, to make the discharge at high-water, or as near high-water as possible, so that it may have all the advantage of the first ebb. This, however, is not always practicable in low-lying districts, unless, indeed, we employ artificial means for raising it ere it reaches the outlet.
Let us now consider the question under another aspect ; supposing the refuse to be turned to acconnt for agricultural purposes : this, although somewhat more complicated, we shall probably find more easily dealt with than the previous case. The question again resolves itself into two distinct parts,— the first where the sewage is used in a liquid state, the other where it undergoes a preparation previous to use, and is applied in a dry state,—and these two will require some what different treatment. The first arrangement, where the manure is expended entirely upon the surrounding neigh bourhood, must depend, in a great measure, upon the charac ter of the neighbourhood, and the comparative levels between it and the town to be drained. In this case it will be advisa ble to divide the town into different drainage levels, cor responding with the general or average altitudes of each distinct level of the adjacent country, and provide each drainage level with a separate outlet, situated as near the corresponding level of the adjacent land as may be in other respects convenient. By so doing we shall dispense to
some extent with the use of pumps, and other expensive con trivances for raising the manure; we shall also save a con siderable amount of excavation, and avoid very large main sewers. It must not be supposed, that here, or in any other places in this article, we lay down fixed rules of universal application, but rather throw out hints which may be found generally useful ; the peculiarities of each particular locality must decide the method of drainage ; there may be some localities where the above directions may not only be unad visable, but simply impracticable, and we only bring them forward that they may not be left out of consideration in any instance, and may be adopted when found to be eligible: It is useless to think of laying down positive rules in such cases, where the nature of the subject to be treated of must neces sarily vary in individual instances.
The last case which we have to consider is, when the sewage is to be applied for agricultural purposes in a dry state, and this will, we think, prove to be the most simple of the three. We have here only to collect the sewage, and conduct it into one or more tanks, in such situations as may be most convenient, where it will undergo a process of dis infection, and will be precipitated, or solidified for the pur pose of conveyance to such lands as may require it, either in the neighbourhood, or at a distance. It will be advisable that the tanks he at sonic distance from the town, to obviate any danger or inconvenience which may arise during the pre paration ; and it is further to be desired, that they should be near a canal, river, or railway, so as to present fheilities for carriage to distant localities ; it would be well if they could be placed in proximity to both rail and river, for while the former will be very convenient for the purpose of conveyance, the latter will be further useful for the discharge of the sewage-water which remains after the extraction of the solid matter, if, indeed, it be not applied to some more useful pur pose. This water may be discharged into the river without inconvenience or danger, for if not already sufficiently puri fied, it would be no difficult matter still further to purify it by filtration. If a ready means of discharge for the super natant water be provided, the size of the tanks may be greatly diminished, and in the same, or rather greater, proportion, the expense and difficulty of their construction.