don basin. They are silicious iron-stained gravels, sands, silt, and peaty deposits lying in grooves or pockets parallel to the river, about twelve to fourteen feet in thickness, but occasionally, though rarely, thirty-five feet thick. They are evidently deposited by the ever changing courses of the river and its tributaries. The gravel deposits being much the older, are found frequently in the higher districts, but they presented no difficulties in the construction of the main drainage works. Not so the sand and the peat, however, as the former was generally found charged with water, in consequence of its being deposited in the impermeable clay pockets in the lower districts, thus entailing a large outlay for pumping; the latter, on the other hand, had to be removed before the foundations of the necessary works could be laid beneath it. The natural surface of London may therefore be considered to be clay, pitted and grooved by the action of the river and its streams; and many of these inequalities are filled in with sand, gravel, peat, and silt.

In the consideration of the second point, namely: "the natural streams and their various drainage areas," it is necessary that the alterations made by modern improvements be left out, and the natural streams of the basin, pur et simple, be dealt with.

The London basin is drained by the River Thames, which has a course through it from west to east, the country on each side gradually rising from the river, north and south, the southern portion being the flatter of the two.

Each of the main portions, viz: the northern and the southern, was intersected by several small streams running north and south from the highlands, and emptying their contents into the river after meandering through the flatter portions of its banks. These streams, and their branches, subdivide the two main areas into minor drainage areas, and collect the drainage flowing east and west. Thus, a section taken through London, and paralle! to the River Thames, would present a series of undulations, as shown in Plan No. 2, Fig. No. 4, in the depressions of which flowed the various streams, or drains, emptying into the river. Many of these undulations have utterly disappeared, but the descent from Holborn and Ludgate Hills, into the Fleet Valley, and that from Piccadilly to Knightsbridge, are very apparent to even the casual observer.

The total area drained by the London Main Drainage, with which this paper deals, is ninety-nine and a quarter square miles, extending from Hammersmith on the west, to Woolwich on the east, and from Stamford Hill on the north, to Anerly on the south. This is divided, as before mentioned, into two portions, the southern having an area of forty square miles, and the northern an area of fifty-nine and a quarter square miles.

The main valley lines, or streams draining the northern portion, were the Brook Green, Counter's Creek, Ranelagh, King's Scholar's Pond, Fleet, London Bridge or Shoreditch, Hackney Brook, River Lea, and several minor brooks and drains. The drainage having been diverted from the River Lea, the area drained by it formerly, now empties its sewage into the Hackney Brook, and others.

The collecting or drainage areas of the above are as follows:

Fleet (anciently called Holebourne or Old Bourne).

London Bridge..........

Hackney Brook....

Minor streams, including portion of the area formerly drained by River Lea.....

Total........

Square miles.

1

21

5

3

61

3

71

301

59

The streams of the southern portion were the River Wandle, Falcon Brook, Heath Wall, Effra, Earl, River Ravensbourne, and several minor brooks, a large area being undrained. The drainage into the Rivers Wandle and Ravensbourne having been diverted, is taken directly by the new main drains.

The drainage areas of these are as follows:

The low-lying grounds, as for instance, at Lambeth, Chelsea, and the Isle of Dogs, etc., were unreclaimed swamps or partially drained by small creeks and ditches.

The third point for consideration is "the early system of artificial drainage adopted in the metropolis." As habitations and streets began to spread themselves over London, it became necessary to provide some system for carrying off the rainfall from the streets by other means than that of the gutter. Brick drains were consequently laid down, leading to the nearest valley line or stream, while those nearest to the river drained directly into it. The general direction of these drains, in order to obtain a fall and to intersect the valley line, was necessarily east and west, following the dip of the land. Those in the low districts were provided with sluices or flaps to keep back the tide as it rose, and so were tide-locked for many hours. As the majority of them delivered their contents at or near low water, many portions of London, but those more particularly on the south side, were subject to constant floods. The household refuse and fæcal matter were disposed of in cesspools,

the liquid portion of it percolating through the soil, and so finding its way into the streams, and thence to the river.

In consequence of the rapid spread of population in London, it was found necessary from time to time to cover in the main valley lines. Then they were inverted, and streets were often formed upon them, but the lower portions of many of them were still left open as before.

Such was the state of the London drainage previously to the year eighteen hundred and fifteen. The ground of the metropolis was honeycombed with cesspits, which saturated it with poisonous matter, giving rise to malaria of various kinds. The higher portions of the town delivered their surface drainage into the main streams, to be by them delivered to the river, while the lower portion delivered direct into the river itself, at low water, as before described.

At about the forementioned date (eighteen hundred and fifteen), an Act of Parliament was procured, making it permissible to drain household refuse, etc., into sewers, and their management was intrusted to eight separate Commissions, each of which had a well defined district under its control, five being on the northern area, and three on the southern. Each commission being independent of the other, no one system of drainage was carried out. Thus the seeds of much evil and expense were liberally sown, by the construction of every conceivable form of sewer, without any regard to the dimensions of those into which they had to deliver, and without any consideration as to the levels of those in the adjoining districts. As the inhabitants gradually availed themselves of the opportunity of draining into the sewers, by making overflows from the cesspits or drains directly into them, the open brooks, or valley streams, became very offensive, and consequently portions of many were covered in, though for many years the Ranelagh continued to discharge into the Serpentine in Hyde Park. A few years ago it was diverted from that lake. It was then found necessary to temporarily drain it to remove the putrefying mud, and to relay the bottom with clean ballast.

In the year eighteen hundred and forty-seven, in consequence of the passing of a new Act of Parliament, the eight Commissions then existing were removed, and in their stead, a Commission for the whole of London was appointed. At the same time, the drainage of houses into the sewers was made compulsory.

The new Commission introduced the system of pipe drainage, conveying the sewage from the houses direct into the brick sewers, as well as pipes of large diameter to drain the streets, instead of the brick sewers heretofore in use. The whole of the fæcal matter was, therefore, carried into the valley lines, and thence into the River Thames.

But now the evil of the badly constructed sewers of the older Commissions began to be apparent. Many of them being large, with segmental inverts and vertical sides, thus giv ng the least hydraulic mean depth, and with small fall, there was little or no flow in them; and the Sewage remaining stagnant, decomposition rapidly set in, giving forth its poisonous gases, until the next rainfall partially cleared it away, carrying the foetid matter reeking to the river to enlarge its sphere of destruction.

In consequence of the connection of the house-drainage with the sewers and main valley lines, it became necessary to have tide-flaps and sluices at the outlets into the river to prevent flooding the dwellings as the tide rose. The flow, under these circumstances, was not more, even in the well constructed sewers, than six hours a day, leaving eighteen

hours during which precipitation of the solid matter could take place. The introduction of water-closets, about this time, augmented rather than lessened the evil, as decomposition was found to take place much more readily in the presence of ordinary water. The cholera, in the year eighteen hundred and fifty-four, consequent on the above state of affairs, cost London over twenty thousand lives. The river had become saturated with sewage. Parliament took up the question, and, after appointing six Commissions, without any practical result, the present Metropolitan Board of Works was formed. It is a representative body, elected from the various districts and parishes of London. Mr. J. W. Bazalgette, now Sir Joseph Bazalgette, C. B., was appointed Engineer to the Board, to carry out the main drainage of London and other public works.

OBJECTS OF THE MAIN DRAINAGE SYSTEM.

The objects sought to be accomplished by the main drainage of London are as follows:

1. To keep the River Thames, in the neighborhood of London, free from sewage at all times of the tide.

2. To abolish all open ditches and cesspools, as well as defective or shallow sewers.

3. To maintain a continual and unintermitting flow in all the sewers along their whole length, with the aid of lifts where necessary, by which evils arising from pent-up sewage, viz: the generation of noxious gases, and the unavoidable formation of deposits in the sewer during its stagnation, would be avoided.

4. To construct the sewers at inclinations, so proportioned to the volume of fluid to be carried off by each that the velocity of the current should keep them clear of deposit without the need of regular periodical flushing, which experience has shown to be not only troublesome and expensive in its operation, but also very injurious to sewers and drains in which it is practiced.

5. To form the main sewers at such a depth as not only to receive the drainage of the deepest existing sewers, but to answer the purpose of main drains capable of extension towards the extremities or borders of their districts.

6. To provide a natural escape direct into the river, by the power of gravity alone, for storm waters and land floods of the covered streams, so as to prevent any needless surcharging of the intercepting sewers with harmless flood water. Also, to construct the new intercepting sewers of such sizes only as would be sufficient to take the general drainage of their districts, including ordinary rainfalls.

7. To follow existing public streets, roads, or paths, so as to avoid heavy compensation for injury to private property, wherever this could be done, without causing injurious curves or undue prolongation of the sewers and consequent loss of gradient.

8. To provide reservoirs at the outlets of the main outfall sewers so as to be enabled to discharge the sewage at the most suitable condition of the varying tide in the river.

9. To carry off as much sewage as possible to the outfalls by gravi tation alone, using the aid of steam-power for lifting the residue.

The foregoing objects have been carefully held in view, and have been practically carried out by the Metropolitan Board of Works.

From the description before given of the relative positions of the main valley lines and subsidiary drains, it will be seen that a series of

main drains laid parallel to the river would intersect the valley lines at right angles, and by being placed at varying levels as the elevation of the land increased, could be made to collect all the drainage of the districts above them, which could then be carried to the desired locality. Such is the scheme adopted.

For each of the main areas, namely: that on the north side of the river, and that on the south, similar systems of drainage are adopted. Three trunk lines, with various branches, are in each case laid down to take the sewage and ordinary rainfall, forming High, Middle, and Low Level Sewers, as shown on Plan No. 3, and receiving the drainage of the districts indicated by their names. The storm waters are, by means of overflow weirs, discharged into or dammed into the old main valley lines, and allowed to flow into the river.

In order to satisfactorily carry out this system, it was necessary to

ascertain:

First-To what distance below London it was expedient to carry the sewage to be discharged into the river, in order that it might not be carried back with the flowing tides, and also at what state of the tide it was most desirable to discharge it?

Second-What is the minimum fall that should be given to the intercepting sewers?

Third-What is the maximum quantity of sewage to be carried away at any given time?

Fourth-What is the quantity of rainfall to be carried off by them? Fifth-What dimensions should be given to them?

Sixth-What pumping power would be required, and the description of engine to be used?

Seventh-What storage would be necessary in the reservoirs?

Each of the above questions involved a considerable amount of study and experiment, and satisfactory conclusions were only arrived at after considerable labor on the part of those engaged in the research, though they were assisted by the most eminent engineers of the day.

Upon the first two points, viz: "to what distance below London it was necessary to carry the sewage," and "what should be the minimum fall given to the intercepting sewers," very much depended. As the farther down the river it was determined to go, or the greater the fall required in the sewers, the smaller would be the area which could be drained without the employment of pumping power; but it was incumbent at the same time to go such a distance as would admit of no probability of the return of any of the sewage to the inhabited portions of the town.

In order to fix upon the position of the outfalls, and the time at which the sewage should be discharged, numerous experiments were made with floats placed in the river at various times of the tides, until at a point near Barking Creek, fourteen miles below London Bridge, where the northern outfall is now situated, the following experiment was made, and recorded in a report by the late Mr. Robert Stephenson and Sir William Cubitt, in eighteen hundred and fifty-four:

"On the thirteenth of July, eighteen hundred and fifty-one, a float was put into the center of the river opposite Barking Creek two hours after high water. This time was chosen because it was found that sewage discharged into the river two hours before high water arrived at about the same point above Barking Creek as sewage discharged two hours after high water did by the next flood tide. At low water the