October 19. Morning, depth of water in pipe 3 inches ; afternoon, depth 2 inches.
October 21. Heavy rain all day ; depth of water in pipe 4 and 5 inches.
October 23. Morning, 3 inches ; afternoon, very heavy rain, when the pipe filled.
October 24. Morning, depth varied from 2 to 24 inches; afternoon, from 24 to 3 inches.
October 25. During the day, depth of water varied from 2 to 24 inches; afternoon, from 21 to 3 inches.
October 26. Morning, depth varied from 4 to 3 inches; afternoon, from 24 inches to 3 inches. During the above three days, the weather was mostly fine. The considerable variations are due to the times of the water being on at the houses ; the sewage at such times is much clearer, as well as increased in quantity.
October 27. On this day a storm occurred, which, for a short period, was very violent ; the waters filled the pipe, and rose above it IS inches, but did not reach the top of the head-wall ; when the waters had reached this maximum height, they receded to nearly the level of the pipe in twenty minutes.
October 28. Depth during day varied from 3 inches to 14 inches.
October 30. Depth during day varied from 2 inches to 24 inches.
October 31. Depth during day varied from 14 inches to 2 inches.
November I. Variation of depth from 14- inches to 24 inches.
November 2. Variation of depth from 2 inches to 24 inches.
Another series of experiments was very carefully super intended by Mr. Lovick, Surveyor to the Commission, in Earl-street, Marylebone : the original sewer presenting a sectional area of 15 feet, and having a flat segmental invert 3 feet in width, in which deposit accumulated at the rate of 6,000 cubic feet in 31 days. The area draining into this sewer was 43.656 acres; and the number of houses drained 1135. Upon the invert of this sewer, a 15-inch glazed stone-ware pipe was laid for a length of 115 feet, and having an inclination of 1 in 153.
From these experiments, it appeared that the average consumption of water was about 5.7 gallons per individual per day, or 51+ gallons per house per day ; the average quantity discharged when there was no rain, and no fresh supply of water, was 444 gallons per house per diem ; and on the days when the water was supplied to the houses, 209 gallons per house per day. Thus the mean flow on the days when water was supplied to the houses by the water companies, was nearly 41- times greater than the mean flow on ordinary days, or days when there was no supply.
The annexed table, deduced from those experiments, will give a correct idea of the several conditions of discharge on ordinary and water days :— The maximum being greater than the minimum, on the water days 140 times, and on ordinary days 26 times. The experiment was continued for 30 days, 16 of which were water days, and the remaining 14 ordinary days, and the period of the year at which it took place was during the months of February and March. The least flow in the above table refers to the least flow which occurred at any one time during the range of observations; the mean least, to the average of the lowest discharges of every day ; the mean, to the average of the mean of all the observations ; so also of the greatest and mean greatest.
As an evidence of the false principles on which the old system was based, the same surveyor states that the sec tional area of the drains in this block of houses was 596 feet, whereas the sectional area of the outlet sewer was only 15 feet, or about one-fortieth the aggregate sectional area of the house-drains. He also states, as the result of his expe rience generally, that blocks of houses drained with 3, 4, 6, and 9-inch pipes, have been more efficiently drained than under the old system.
The same opinion, with reference to the size of sewers, was given by Mr. Phillips, also Surveyor to the Metropo litan Commission and the late Westminster Commission, in his examination before the Sanitary Commissioners ; but he considers a constant supply of water indispensable, if smaller sizes be adopted. In answer to the question " Have you at all considered the capacities of sewers necessary for draining the different areas of ground ?" he replies, " Yes ; I have given the subject much attention. If
the consideration of the sizes of sewers was confined solely to the carrying off the water supplied by the several water companies, then I apprehend that pipes somewhat larger in size than the supply-pipes themselves, would suffice ; but provision has to be made for receiving and carrying away the waters of heavy rains. In London, continuous heavy falls of rain are not of long duration, lasting seldom more than from one to four hours. About one-fifth of the quan. tity that falls, is absorbed partly by the dryness of the sur face of the roofs, the paving, and the ground, and partly by the porosity of the ground itself. A farther proportion is also prevented from flowing to the drains and sewers at all by hollows in the surface, and again reascends into the atmo sphere as vapour. There is also a small quantity that enters into the composition of animal and vegetable bodies. Then there is the resistance the flow experiences from the friction of the entire surface, being accelerated or detained in propor tion as the surface is more or less inclined. To provide for the discharge of a fall of rain of two inches in depth, has been considered by Mr. Hawksby, C. E., the extreme datum upon which to proportion the capacities of town-sewers generally. Now I believe that, practically, the sizes in his table, although they may appear theoretically correct, are (excepting for the smallest sizes) too large for sewers in London. It is extremely violent rains alone that produce a depth of two inches per hour, and such rains occur only once in four or five years, if so much. I am of opinion that it is unnecessary to proportion the size of the sewers to meet an extraordinary occurrence that may probably happen only once in so many years. My reason for not fearing any seri ous damage from an excess of rain at remote intervals, being provided for in surface-channels, excepting perhaps in situ ations peculiarly liable to inundation, (for instance, at the foot of a long or steep declivity, or where the waters may from any cause be suddenly congregated at one focus,) is that I have observed, that in towns entirely destitute of underground drains, no such inconvenience is felt as would justify the formation of enormously large sewers, or the ex penditure of large sums of money to provide against it." The evidence of Mr. Roe, late chief Surveyor to the Metropolitan Commission, and previously to the Holborn and Finsbury Sewers, will be deemed most valuable, if not conclusive on this subject, both on account of his long expe rience, his straightforwardness, and unbiassed judgment ; he was also the first to pay attention to the subject, and introduced many improvements, before it had become the topic of general consideration. In his evidence before the Sanitary Commission, he states as the result of his experience, that a cylindrical sewer, 4S inches in diameter, with a fall of 1 in 240, is sufficient to drain 100 :.ores of town-area, allowing for a fall of rain unusually large. Aiso that in a street 924 feet in length, containing 03 houses on an area of 6 acres, 1 rood, S poles, a fall of rain 2 inches in depth, producing 346 cubic feet of water per minute, would require a sewer of 2.44 feet capacity, with an inclination of 1 in 4S0. Allowing further for a supply of water of 75 gallons per day to each house, he would add to the capacity of the sewer .10 of a foot, making a total of 2.6 feet. With respect to uncovered land, he gives as the result of five months' observation, that the greatest amount that was found to reach the invert from a fall of rain of finch in the hour, averaged 3 cubic feet per acre per minute at the period of the greatest flow, and that that period was generally from the greatest to one hour after the heaviest port-ion of the rain had fallen.