ence, those of the upper half, if laid parallel with the face, will be in the best direction; and the almost vertical direction of the hydraulic pressure will simply tend to compress the bank, and neither to overturn it nor slide it on its base.

While giving all credit to Mr Aytoun for the correctness of his views, your committee beg to report that, in so far as the idea of applying the puddle to the slope, instead of in a central trench, is concerned, this has been already practised; they understand that it is not uncommon in America. And in this country Mr Robert Thom read a paper in June 1843 before the Institution of Civil Engineers (Minutes of Proceedings, vol. ii. p. 183), "On the Formation of Embankments for Reservoirs to Retain Water," Mr Thom puddles both slopes as the work proceeds, and does not approve of a central puddle wall or trench. The puddle he was in the habit of using was compounded with a view to keep back moles and vermin, and consisted of clay, mixed with stones of about eight cubic inches each.

Your committee think with Mr Aytoun that there is room for censure in the dangerous manner in which embankments are sometimes carried on. They refer particularly to the fact, that the pipe or conduit by which the water may be taken out of the reservoir, is frequently quite inadequate to carry off flood waters, so that, while the embankment is in course of construction, an unusual duration of very wet weather may cause the water to rise so high as to overtop and destroy the incomplete earthwork. If a proper plan were pursued, a very small outlay would obviate this gambling with the lives and property of others.

ROBERT H. Bow, Convener.
R. W. THOMSON.

JOHN REID.

39

On the Utilisation of Sewage, with a Description of the Plan of Messrs Napier and Hope for the Utilisation of the Sewage of London. By GEORGE ROBERTSON, M. Inst. C.E., F.R.S.E., &c. Illustrated by a coloured Plan.*

At a recent meeting of the Society, at which our President read a paper in connection with the pollution of rivers by sewage and other matters, allusion was made to the scheme of Messrs Napier and Hope for the utilisation of the sewage of London. I then mentioned that Mr Hemans, the engineer who laid out the scheme, had paid a recent visit to the irrigated meadows near Edinburgh, in company with Professor John Wilson and myself; and, during that visit, had given me such information regarding his London plan, as to enable me to give the Society a somewhat detailed and authentic account of it. He has since sent me the plan which illustrates this paper, and which will greatly elucidate and simplify my task. I have also been furnished with pamphlets and other documents on the subject. It is one which cannot fail to be interesting to this Society, for the whole question of the utilisation of sewage, taken in connection, as it must ever be, with the pollution of rivers, is one of the most important questions of the day; and will doubtless occupy a large share of the attention of Parliament for several sessions to come. The necessity for some legislation about it has become gradually more and more urgent since the introduction of the water-closet system.

Before describing the plan for the utilisation of the metropolitan sewage, it may be as well to glance generally at the present state of the question. It is now universally admitted, that the method tried at Tottenham, Leicester, and elsewhere, of precipitating the solid matters in sewage by lime or other cheap substances, has not been a commercial success. At least five-sixths of the valuable matters contained

* Read before the Society 27th February 1865, and awarded the Society's Silver Medal. Since this paper was written, the scheme for the utilisation of the sewage of London has passed the Committees of both Houses of Parliament.

in sewage are not capable of being precipitated by any known process, whatever chemistry may do for us in the future.

The sewage must therefore be used as a fluid; and though it might be desirable to obtain it in a more concentrated form, by separating the house-drainage from the surface-drainage of a town, my own impression is, that we must accept the present state of matters. Civilisation will not permit us to withdraw from the water-closet system, now generally introduced; and it is not likely that we shall ever have our streets complicated with a double system of sewerage, especially after the large sums of money which have been expended in perfecting the present arrangements throughout the whole kingdom.

We must therefore use our sewage in its present liquid and diluted form. Can it be used with advantage to the produce of the land? and can it be used with profit on a reasonable outlay? My firm belief is, that the sewage of almost every town in this country might be more or less used with advantage for the irrigation at least of meadow grasses and Italian rye-grass; and, in most cases, with fair prospect of pecuniary profit, if properly done.

There may, however, be instances, especially in towns far from the sea, or in deep valleys, where a pecuniary loss will have to be faced, as part of the expense of draining the town; for the day cannot be far distant when every town will have to adopt some means, either of interception, or of clarifying its impure water before permitting it to run into the adjacent stream.

I believe that, almost without exception, every town which can use its sewage for irrigation by means of gravitation alone, would find the necessary outlay to be amply repaid. Where gravitation is impossible, many towns might make use of water-power for pumping; and even where steam has to be employed, this need not necessarily discourage; for pumping by steam, as I shall show, is not such an alarmingly expensive thing, and its cost is well known.

There are two main points in the sewage question, as it stands at present, upon which there is a great difference of opinion-the quantity to be applied per acre-and the value