Mr. Roe's observations on the flow of water in the Fleet Sewer are of great practical value. This sewer, at its out let into the city, is 12 feet high by 12 feet wide, with a super ficial area of 120 feet ; it receives the discharge of 60 sewers, the total sectional area of which amounts to about 550 feet, and its inclination varies from 1 inch in 100 feet to 1 inch in 2 feet, while some parts are on a level. The area drained by it consists of 1,181 acres of towmarea, and 2,656 acres of rural district. During the unusually heavy thunder-storm which occurred on August 1, 1846, the area of flow at this part of the sewer measured 106 feet ; in a heavy thunder storm in July, 1S-14, the sectional area of flow was 79 feet, and on an extraordinary thaw, after 611 of snow in 1S-11, the area occupied was 54 feet, while the ordinary flow does not cover an area of 10 feet.
The following table, made under the directions of the same gentleman, are from gaugings of several sewers under his charge, and will give a fair notion of the true state of the ease: In opposition to this evidence, we have the authority of such names as Walker, Cubitt. and Brunel, names which may not be passed by unnoticed, although with the previous statements before us, it seems sAinewt•it difficult to coincide with their views. While, however, we pay due deference to gentlemen of such high standing in their profession, we must not forget that such men as Mr. Roe have probably had the greater experience in this p•tioular class of works. Let us attempt to simplify the question as much as possible.
The objections raised against the old system 1st, that they arc unnecessarily expensive-2ndly, that they are inef ficient Their great expensa arises from their size, therefore we have to show that their size is unnecessarily large. That they are larger than required for the ordinary drainage can not, we imagine, be a matter of question, after the result of the above observations and experiments have been made known ; nor can we attempt to disprove the unvarying evi dence on this point, given by persons practically acquainted with the subject ; the table of gaugings by Mr. Roe seems to decide this question, for during the extraordinary storm, in 1846, we find that by fir the majority of the examples were not full on that occasion, nor, indeed, nearly so. Out of 16 examples only two were filled on this occasion, one of which was a main line of 13-feet capacity, draining an area of 110 acres, and the other a 15-inch drain, relieving an area of 0.84 acres. During the experiment in the Earl-street sewer, which happened in a wet season of the year, a 15" pipe was found more than sufficient for the drainage of 44 acres of covered ground, with only one exception, which occurred during a heavy storm, when the discharge amounted to above 300 cubic feet per minute. In the experiments in Upper George-street, during a continuance of 96 hours' rain, the 12 inch pipe was never observed to be more than half full ; but on one occasion, during a violent storm, the water rose 18 inches above the pipe ; this state of things, however, lasted but a short time. The area drained by the Upper George
street sewer was also 44 acres.
Now, putting aside for the present the question as to whether the pipes used in the above experiments were of sufficient capacity, Mr. Roe's experiments, which give, we think we may say, the very greatest amount of discharge which can ever take place, are decisive thus far—that by far the majority of existing sewers are very much larger than can at anytime be necessary; and with respect to the exceptional two, it remains a question, whether under an improved system of drainage they would ever have been put to such a test, but under the old system, when, occasionally, a sewer got smaller towards its outlet, the levels were ill-arranged and defective, &c., it is not to be wondered at, that sometimes one sewer had .
to perform more than its fair share of work, whilst others were almost inactive; if the sewage had been more equally tributed, no one sewer would have been so much overcharged.
Taking then the maximum quantity ever discharged, as the rule to guide us in fixing the proper dimensions of sewers, we find that the old system is too capacious; but beyond this it is questionable whether it is necessary or advisable to provide for such extraordinary flow, and this question becomes of more importance, if it can be proved that extra capacity is detri mental to the efficient discharge of sewage matters. In this state of the case, the real matter at issue is, whether it be preferable to be subject to a constant inconvenience, or a tem porary and problematical one. Some say we must provide for all emergencies ; others would rather rim the risk. We are inclined towards the latter opinion. But there remains another argument in favour of large sewers, and that is, that they admit the passage of a man, for the purpose of inspection and periodical cleansing, which the advocates of the old system deem to be necessary ; if, however, it can be proved that smallersewers will keep themselves free from deposit by the extra velocity and scour of the water, this objection will be removed. Again, this objection will not hold good against the substitution of very small pipes for those sewers which, although much larger than requisite for the ordinary flow, cannot conveniently be entered for this purpose; for if the work causes great discomfort to the labourer, it is certain that it will not be performed efficiently, if indeed, it be per formed at all; we should imagine that sewers less than 4 feet in height, are not inspected much more frequently than a 15-inch pipe would be.