drained. With regard to culverts, a settlement or slip of an embankment over a culvert may unequally strain, fracture, and displace portions of it, and therefore interrupt the flow of the drainage waters, which may then reach the seat of an embankment and cause it to be in a dangerous condition. They are usually necessarily placed at the deepest point of an embankment, and consequently the most difficult to make repairs. In such a situation they should be built sufficiently large to allow of the easy passage of a man, in order that due inspection may be made, and be constructed of materials of a durable character. Should a naturally firm bank exist on one or both sides of a stream, it should be stripped of all plant growth and decaying matter, and be preserved in order to form a natural wall to relieve a culvert from side pressure, but firm or hard material must be inserted between the back of the wall and the face of the stream bank so as to support the wall against the pressure it receives from the arch. This leads to a consideration of the best form of culvert. In clay soils, and those which exert pressure from expansion, especially if the culvert is surrounded by clay earth, the circular is generally considered to be the best form, and this has been proved to be so in tunnels in similar soil, with splayed wing-walls to assist and guide the flow and help to keep a clear entrance. In granular soils, such as dry sand or gravel, the earth acts differently and more in accordance with the angle of repose theory of pressure, therefore the strain upon the arch would be the greatest, and its thrust must be counterbalanced at the sides, the strain upon the invert being probably very little and due to the tendency to an overturning movement of either of the straight walls. However, in culverts as in tunnels, it is impossible to say the exact amount or direction of the strain, although it may be approximately computed. Should it be necessary to erect a culvert upon soft ground, as much of it as practicable should be excavated and a concrete foundation be placed thereon; it is also advisable to allow an extra length to that required by the calculated slope. A culvert should have an invert unless upon a hard rock bed, and care should be taken that there shall be no leakage at the springing of the inverted arch or under it, or beneath the sidewalls at the level of the surface of a flat stone drain or below it.