Again: where land to be drained is part of a large sloping tract, and water runs down, at certain seasons, in large quantities upon the surface, an open catch-water-ditch may be absolutely necessary. This condition of circumstances is very common in mountainous districts, where the rain which falls on the hills flows down, either on the visible surface or on the rock-formation under the soil, and breaks out at the foot, causing swamps, often high up on the hill-sides. Often, too, in clay districts, where sand or loam two or three feet deep rests on tough clay, we see broad sloping tracts, which form our best grass-fields. If we are attempting to drain the lower part of such a slope, we shall find that the water from the upper part flows down in large quantities upon us, and an open ditch may be most economical as a header, to cut off the down-flowing water; though, in most cases, a covered drain may be as efficient. At the outlets, too, of our tile or stone drains, when we come down nearly to the level of the stream which receives our drainage-water, we find it convenient, often, and indeed necessary, to use open ditches--perhaps only a foot or two deep--to carry off the water discharged. These ditches are of great importance, and should be finished with care, because, if they become obstructed, they cause back-water in the drains, and may ruin the whole work. Open drains are thus essential auxiliaries to the best plans of thorough drainage; and, whatever opinion may be entertained of their economy, many farmers are so situated that they feel obliged to resort to them for the present, or abandon all idea of draining their wet lands. We will, therefore, give some hints as to the best manner of constructing open drains; and then suggest, in the form of objections to them, such considerations as shall lead the proprietor who adopts this mode to consider carefully his plan of operations in the outset, with a view to obviate, as much as possible, the manifest embarrassments occasioned by them. As to the location of drains in swamps and peculiarly wet places, directions may be found in another chapter. We here propose only to treat of the mode of forming open drains, after their location is fixed.