We have now to consider whether drainage may be more efficiently discharged by a small than a large sewer. The small quantity of water ordinarily passing through the large existing sewers, is allowed, on all hands, to be very sluggish in its movement ; and that a deposit of solid matter is always going on, and that to a considerable extent, is evident from the fact, that large quantities of solid matter are obliged to be periodically removed by flushing or hand-labour, and in several cases when old drains have been opened, they have been found to be almost choked up by such accumulations. It is certain, therefore, that large sewers do not of themselves efficiently remove the refuse matters. This defect was observed some years since by Mr. Roe, and the cause being determined upon, it was attempted to remedy it by reducing the width of the invert of the sewer, and thus narrow and deepen the channel for the water, thereby producing less fric tional area, a greater depth of water, and consequently a greater velocity and improved scour. This practice was found to succeed, and was afterwards adopted in other locali ties. It may be noted in passing, that where much deposit has taken place, it is observed ;hat the stream forms for itself a sort of gut or narrow channel, thus giving visible evi dente of the requirements of nature in this particular.
As the question relating to the velocity and scour of a body of water in various-sized channels was of considerable importance in determining upon a system of drainage, a series of experiments upon the flow of water through various-sized pipes was made, under the direction of the Metropolitan Commission ; and amongst the results given, we find that a 3-inch glazed stone-ware pipe, 50 feet in length, with an inclination of 1 in 120, and being fully charged at the head, will discharge 100 gallons of sewage water in 3 minutes ; and that a 4-inch pipe under precisely similar circumstances, will discharge 200 gallons in the same time ; and further, that such a flow is sufficient to remove any, and even more than ordinary and usual semi-fluid deposit, such as is usually found in house-drains. A mixture of sand with water, in proportion of from to to the volume of water, was also removed. Also that the hydraulic mean pipe of it 3, 4, 6, and 9 inch pipe, when half full, is respectively .749, 1.00, 1.5 and 2.18 inches, and that the fractional line, under the same circumstances, would be 4.71, 6.28, 9.42, and 14.13 inches respectively. Further, that 1 gallon of water through a 3-inch pipe, moved 1 lb.; through a 4-inch pipe, 3 lb. ; and through a 6-inch pipe, 4 lb.; and that 3 gallons of water will carry off 1 lb. of solid feces through a 6-inch pipe with a fall of 1 in 10 ; but to make these results of use, we ought to be acquainted both with the fall and velocity. Another statement is this : if 81 seconds suffice for the discharge of 50 cubic feet of clean water, 84 will suffice when 40 of solid matter is added, and 91 seconds when the solid matter amounts to A.
Such results are generally in favour of the small pipes, hut it is to be regretted that the experiments were not carried on on a more extensive scale, for they afford little informa tion respecting the larger kinds of pipes. It will be generally allowed, however, that a concentrated body of water will move with greater velocity than the same amount where spread over a large surface, the resistance offered by friction will be less elliictive in retarding its progress, and the greater depth afforded by narrowing the channel will tend to improve the scour. The scour will depend both upon the depth and velocity of the stream, and will therefore in both ways be more efficient in a contracted channel. On these two pro perties, depend the efficiency of the drain ; for if the velocity be greater, the discharge will also be greater, and the power to keep the sewer free from deposit will increase in like manner ; for, in the first place, there will be less tendency to deposit ; and in the second, when a deposit takes place, it will be more readily rt. moved. This is both reasonable in theory, and has been proved in practice, for the evidence of Mr. Loviek and others, who have had opportunity of examining the state of small pipes after they have been in use some time, goes to prove that they keep themselves almost perfectly free from deposit. It will be perceived by the following table, prepared from the experiments made in the Earl•street sewer, that the quantity of solid matter in suspension in sewage-water, bears but a small proportion to the liquid, and it is therefore probable, that with a fair velocity of flow there would be but little tendency to deposit.
The principal objections against the use of small-pipe sewers are : 1st. That if any obstruction occur in the pipe, so as to cause accumulation of deposit, the pipe rapidly becomes completely stopped up, whereas, in the larger sewers, even though obstructions may occur, they will not entirely stop the flow, or at least the accumulation must pro ceed for a considerable period ere the entire area be filled up. 2nd. That if' such stoppage take place, there is no means of ready access to the sewer to examine it, and that the road will probably have to be broken up in several places ere the drainage be repaired ; and even then it is almost impracticable to reinstate the pipes in the same conditions as when first laid down. 3rd. That owing to the nicety required in lay ing the pipes, an uniflirm inclination is not easily preserved, nor is there any satisthetory means of knowing when the invert is perfectly smooth ; that if the pipes be defective in form, or imperfectly laid, or if the eement with which the joints are made be allowed to project within the pipe above the general surface. an obstacle is at once offered to the flow, which is not unlikely to cause deposit., and thereby a stop page in the pipe.