generally most necessary at the upper and higher levels. Sewage gases, being lighter than atmospheric air, flowed upwards, while the sewage itself flowed downwards, and as the velocity of the discharge of sewage water downwards would be in proportion to the gradient of the sewer, the flow of sewage gases upwards would also be in proportion to the gradient. Therefore, a town having a naturally steep gradient, required the fullest means of ventilation; and so far from its being easy to sewer such a town, he held, if proper means were not taken to provide for ventilation, the Engineer was liable to leave that town worse than he found it. Within his own knowledge, disease had broken out in well-sewered districts, which was traced to a connection of house drains with unventilated sewers; and in those cases the highest portions of the towns suffered most severely. He thought the authorities at Dundee should not rest satisfied with what had been already done, nor until the work was completed by a proper ventilation of the sewers. Mr. Fulton seemed to infer, that sewers which wanted ventilation had been badly constructed. Mr. Rawlinson, on the contrary, held, that if such sewers were sweet, they must have been badly constructed; and that there were joints and crevices which allowed the gases to escape; because, with a perfectly tight sewer, the gases must make their way to the upper extremity, and have a tendency to find exits into houses through the house drains. The method of cleansing sewers, proposed by Mr. Fulton, was very similar to a system invented by the late Mr. Richard Blades, and used by him at Liverpool; it had also been introduced to the notice of the Metropolitan Board of Works. He thought it right to mention this, because credit ought always to be given to those who had the priority in any useful invention.

Mr. Alfred Williams had presented an able description of good and useful works executed at Newport, Monmouthshire, where ventilation had been adopted. He did not quite understand what was meant by the remark, that it was not considered advisable to use stoneware pipes as main sewers, but that it was preferable to adopt a sewer of sufficient size for men to traverse. He thought any Engineer in sewering a town, ought to banish from his mind, at once and for ever, the necessity of constructing sewers sufficiently large for a man to pass through, unless those dimensions were required to discharge the sewage flowing into them, and should rather endeavour to design such a form of sewer as would be absolutely certain to discharge the contents derived from the upper extremities and from all the tributaries, and to keep the sewers clean and sweet. He could mention instances, in his own practice, where sewers had been constructed so as to act in the most efficient manner, without incurring the additional cost of making them larger than was necessary, merely

for the sake of saying at the conclusion of the work, "that they had a sewer large enough for a man to go through." He could point to dozens of miles of sewers which had been in operation a sufficient length of time to subject them to the severest test, without any instance of failure from choking up, or requiring handcleansing. The sections of the Newport sewers (Plate 4) were certainly of the most approved forms, and the bricks employed there were well adapted for the construction of such sewers with economy. Looking at the section, Fig. 8, Plate 4, it appeared that the internal dimensions of that sewer were 4 feet 6 by 3 feet, and that the brickwork of the invert and of the sides consisted of a double ring, 9 inches in thickness, while in the crown there was a single ring only, 6 inches in thickness. He did not know what plan had been followed at Newport, but in his own practice he should have set the two top courses of the thicker part of the work as headers,' so as to bond the two lower rings together, and to give a fair start for the single ring. In fact, in every instance where there were two rings of brickwork in the invert and sides, he insisted that there should be two rows of 'headers' at, or about the springing, so as to bond and tie the work together, where the arch commenced. The method of ventilating a sewer at Newport was good as far as it went; but he thought it was imperfect in this respect, that the manhole shaft was not available, without removing the flag which covered it, and thus breaking up the street.

The system of ventilation he had adopted was shown by Fig. 5, page 288. There was a side chamber; but instead of covering the manhole shaft with a flagstone, which was difficult and expensive to remove, when access to the sewer was desired, it was finished with a manhole cover, which formed the street surface, and did not allow air to pass out; so that the sewer gas was compelled to rise through two, or more charcoal screens, or filters, before passing into the side chamber, or ventilating shaft. Any dirt from the street, falling into the ventilating chamber, was caught in an iron box, at the bottom of which there was a slide, so that the dirt could be readily removed whenever necessary.

In a hilly town the whole system of sewage should not be so connected together as to permit of the accumulation of the sewage gases in the higher parts of the district. Where the gradient was steeper than 1 in 200, and a sewer was, say, one mile in length, it should be divided by steps, or falls, and there should be a flap valve upon the upper, or higher discharging end of the sewer; with at least two ventilating grates at that point, so as to insure thorough ventilation.

It was to be remarked, that the systems of sewerage, both at Dundee and at Newport, discharged into rivers. It was fre

quently necessary to carry the outlet of a sewer to a river, having a low summer level, and a high, or flood level. It was

Manhole, Tumbling Bay, and Double Ventilating Arrangement.

neither advisable, nor always practicable, during flood times, to carry the whole of the sewage, if the quantity was considerable, down below the summer level. Figs. 6 and 7 (p. 289) showed the arrangement he had executed at Windsor Castle, where formerly

a large brick sewer discharged itself into the Thames, close to the Queen's private walk, and was a source of nuisance. By fixing a cast-iron pipe at the bottom of the sewer, and putting a slight dam in front of the junction, all the ordinary sewage was discharged by the iron pipe below summer level, so that the sewage was never thrown into daylight. The brick sewer above took away all excessive storm waters, and there was no danger of bursting, or other difficulty. He found a sewer 4 feet high and 3 feet wide might be required for flood, or storm waters; but that an iron pipe 12 inches diameter would probably discharge the ordinary flow of sewage for three hundred days out of the year, and when there was anything more than the pipe would take off, it would be in extreme dilution, and the river itself would be in flood, so that there would be a minimum amount of nuisance.

The main outlet sewer at West Ham (Fig. 8) was 4 feet 6 inches in height by 3 feet in width. It had a cast-iron invert,

Fig. 8.

which was laid for about half a mile, at the level of low water of spring tides, and then for 2 miles with a fall of 3 feet per mile. Of course the outlet was obtained by pumping. The surface of the ground, under which that sewer was laid, varied from 10 feet to 12 feet below high water in the Thames, and the subsoil was so porous, that with a portable engine within 10 feet of the work, the water could not be pumped below the line shown on the section (Fig 8). Consequently, he had to devise an iron invert, which the workmen could put into its place in the trench under water, by feeling only. The arrangement of the joint was such that there was a certainty of the plates falling into place, and each could be raised by the side flanges, and be rammed below with gravel, or clay. Upon this invert plate cement blocks were laid to above the water line, and the other portions of the sewer were built in the ordinary manner. About 3 miles

INVERT JOINT

of that sewer had been in use for nearly two years, and there was no difficulty in keeping it clean. The velocity of discharge in that sewer, which had its invert on a level, was higher than that which had been given in the Paper on the Drainage of Dundee, for a depth of 2 feet of water and a gradient of 1 in 400. The velocity was due to the surface gradient, or head. Means of flushing were provided, and the only fall was the grade of the surface.

The mode of forming sewer junctions by bent pipes was, he thought, defective. A better plan was at the option of the Engi