We have now considered the subject as regards the three different methods of disposal of the refuse most likely to occur in practice. We must again express our regret that greater labour has not been bestowed on the question of the practicability of the two last methods, yet at the same time we must take occasion to remind the reader, that even had this question been generally disposed of, it would not follow that any particular practice would be advisable in every individual case. The proper determination of the method to be adopted, must be the result of a careful investigation of the nature of the several districts to be drained ; one method may be convenient in one place—another, a different one, in a second. We have here considered the subject solely as regards the disposal, and principally with reference to outlets, as these are the most affected by the various methods of disposal ; let us now turn to the principles of town-drainage generally.
The first thing to be done in laying out a system of drain age, is to prepare a carefully surveyed map of the district to be drained, showing at least the main levels : at the outset, perhaps a block plan will be sufficient ; but before the ques tion is finally determined upon, and details entered into, it will be necessary to have the blocks filled in. To do this, it will be requisite that the map be on a sufficiently large scale ; that adopted by the Ordnance Survey, live feet to a mile, or one somewhat larger, will be found convenient, and on this, additional levels must be laid down before entering into detail. It will be advisable ere commencing the con sideration of the subject, to prepare from this a map on a reduced scale, containing in addition some considerable por tion of the surrounding country, so that the consideration may not be confined to a limited space, without reference to the circumstances of the more immediate neighborhood ; by so doing, much future trouble and expense may be avoided ; by all means let the consideration of the subject be compre hensive. It will greatly assist, if the contour-lines be laid down on this map, so as to afford at a glance a tolerably accurate idea of the nature of the ground. It will be further necessary to obtain information respecting the geology of the district, as well for matters of construction as for other pur poses, and also some general notion of the nature of the soil with reference to agricultural matters, in order to determine the extent to which the town-refuse may be employed as manure in the immediate vicinity. These, however, and other matters necessary to be known, will naturally suggest themselves to those engaged in such works.
The surveyor having made himself thoroughly acquainted with the character of the district to be drained, will be in a pro per position to determine the system upon which to commence operations. The practice of our predecessors has been, to
collect the sewage matter, and convey it by the shortest route to the nearest river, into which it was to discharge ; this done, their object was attained ; the matter was left to remove itself, or deposit on the banks or bed of the river, as best it might. This method is now generally allowed to have been based upon falso principles ; but it must at the same time be confessed, that to a certain degree it did follow out the course suggested by nature. That the drainage should be carried towards the river, is not to be objected to ; for it amounts to nothing more than conveying it to the lowest level of the district; it is the very course which nature adopts, for rivers and streams are indeed the drains laid out by nature ; the only mistake consists in polluting these waters with matters which are prejudicial to health ; had we only to deal with surface-drains, the system would be rational enough. Some persons, because this method has been abused, have thought it right to discard it altogether ; we cannot follow them thus far; if the area to be drained presents, throughout one general slope towards the river, we are inclined to think that this would be the most suitable method of drainage, having the outlet or outlets at the lowest level. If the sewage is to be supplied in its liquid state to the surrounding country, it may be advisable to intercept a por tion cre it reaches this point, but this the circumstances of each case must decide; by such means we should of course be better able to irrigate the higher lands, and should also effect a saving in the size of our sewers, for the lower branches will not have so large a quantity to discharge.
If' again the levels of the main be greatly diversified, or especially if it consist of basins surrounded by higher ground, it would appear advisable, under any of the above conditions, to provide a separate drainage for each level, so as to avoid the great depth of excavation which would be necessitated, were the drainage of the higher levels to be connected with a main which had already received that of a lower level. The better plan would be to convey this line through the lower levels to its outlet, and to form a separate system for the higher grounds. Whether it would be advisable to drain all the low levels together by one trunk-line, and all the high levels by another, or to drain each level separately, must be determined by the circumstances of each special case ; as we said before, no fixed rules can be laid down, we can only pro pose useful hints to be applied as the nature of each case suggests.