The theory is that the iron exists in the soil, and is held in solution in water as a protoxide, and is converted into per-oxide by contact with the air, either in the drains or at their outlets, and is then deposited at the bottom of the water. In a pipe running full there would be, upon this theory, no exposure to the air, which should form the per-oxide. In the case stated, it is probable that the per-oxide is formed at the exposed surface of a large cask, at the spring, and is carried into the pipe, as it is precipitated. Common drain pipes would be full of air, which might, perhaps, in a feeble current, be sufficient to cause this deposit. Occasionally, cases have occurred of obstruction from this cause, and whenever the signs of this deposit are visible about the field to be drained, care must be used to guard against it in draining. To guard against obstruction from per-oxide of iron, tiles should be laid deep, closely jointed or collared, with great care that the fall be continuous, and especially that there be a quick fall at the junctions of minor drains with mains, and a clear outlet. Mr. Beattie, of Aberdeen, says: Before adopting 4 feet drains, I had much difficulty in dealing with the iron ore which generally appeared at two to three feet from the surface, but by the extra depth the water filters off to the pipes free of ore. Occasionally, iron ore is found at a greater depth, but the floating substance is then in most cases lighter, and does not adhere to the pipes in the same way as that found near the surface. Arrangements should also be made for examining the drains by means of wells, and for flushing them by holding back the water until the drains are filled, and then letting it suddenly off, or, by occasionally admitting a stream of water at the upper end, when practicable, and thus washing out the pipes. Mr. Denton says: "It is found that the use of this contrivance for flushing, will get rid of the per-oxide of iron, about which so much complaint is made." _Obstruction by Filling at the Joints._ One would suppose that tiles might frequently be prevented from receiving water, by the filling up of the crevices between them. If water poured on to tiles in a stream, it would be likely to carry into these openings enough earthy matter to fill them; but the whole theory of thorough-drainage rests upon the idea of slow percolation--of the passage of water in the form of fine dew, as it were--through the motionless particles which compose the soil; and, if drains are properly laid, there can be no motion of particles of earth, either into or towards the tiles. The water should soak through the ground precisely as it does through a wet cloth.