The objection to leakage is twofold: sewage matters escaping into the soil might contaminate wells and springs; and it would also rob the flow through the pipes of water needed to carry forward the more solid contents. The continued efficiency of these small drains for carrying away the solid or semi-solid outflow of the house is dependent very largely upon the presence of sufficient water to create a scouring current. While eight-inch pipes are admissible as a safeguard against imperfect laying, they are liable to the grave objection, that, where the service to be performed is greatly less than their capacity, the stream flowing through them will not be sufficiently concentrated to carry forward the more solid parts of the sewage. Up to the limit of their capacity, six-inch pipes properly laid are greatly to be preferred, as insuring a deeper stream which will more generally attain the velocity of three feet per second, needed to move the heavier constituents of the sewage. The difference in cost between six-inch and eight-inch pipes will be sufficient to cover any extra cost of the most careful workmanship. However much attention may be given to the cementing of the joints, it will be impossible to prevent the running into the pipes of a certain amount of mortar; and the workman should have a swab or a disk of India rubber of the exact size of the bore of the pipe, with a short handle attached to its middle, to draw forward as each joint is finished, and so scrape away any excess of mortar before it hardens. Wherever it is, or may probably become, necessary to attach a house-drain or land-drain, there should be used a length of pipe having a side branch, oblique to the direction of the flow, to receive such connection. The location of these branches should be accurately indicated on the plan; and they should be closed with a flat stone or a bit of slate, well cemented in place.