There is ordinarily a ditch around the outside of the hearth jacket extending from just above the base of the columns, which are bricked in solid, up to about a foot above the hearth level, and practice in regard to this ditch has varied greatly. It is customary to use this to carry off the waste water from the tuyeres, coolers, etc., and formerly these ditches were kept full of water to their tops, with the idea that this furnished effective cooling of the hearth jacket and tended to prevent breakouts. But in order to keep the ditch full of water its outlet must be dammed, and when this was done if a breakout did occur the molten iron filled up the ditch to the level of this dam, part way or even all the way around the furnace, and when cold had locked itself between the columns and the hearth jacket, whence it was almost impossible of removal.

At one time the practice prevailed of filling the ditch to the top with large lumps of limestone piled in as loosely as possible, then if a breakout occurred the decomposition of the limestone by the heat and the subsequent slacking of the lime tended to crack apart the mass of iron, which, of course, was much reduced in quantity and solidity by the presence of the lumps themselves. This practice is an improvement, but the practice of carrying the ditch full of water is itself going out of vogue to some extent, and personally I would never have the water in the ditch more than a couple of inches deep. This enables the ditch to be kept open to the bottom, and if a breakout occurs the molten iron will run out the outlet and leave the ditch comparatively free. Moreover, more effective cooling may be obtained by the spray from a small circle pipe causing a thin film of water to trickle down over the hearth jacket than can be obtained with a large body of standing water. This has often been established and is another illustration of the principle that rapid circulation has more to do with heat conduction than the amount of surface and conductivity of the material involved.

Allowing the outlet from the ditch to carry off the iron in case of a breakout has some disadvantages because the iron itself will run along the bottom of the ditch underneath the water for long distances, even in quite small quantities. I have known it to go 60 or 70 ft. and plug up the screen over the waste opening into the sewer so as to create a fearful nuisance.

To meet this situation two expedients are available, one of which is extremely simple and should always in my judgment be used. This consists in having an auxiliary outlet from the ditch normally kept closed and designed to discharge at some point sufficiently removed from the furnace so that any iron carried off by it may do as little harm as possible. This ditch may be closed either by a hand-operated gate or by a thin lead plate. The latter will cut off the flow of water effectively, but will melt in case iron strikes it and open up the emergency outlet. This should preferably be placed lower than the main outlet so as to drain the iron completely away from the latter. The use of this expedient once in my experience saved a shutdown of many hours.

The other expedient is somewhat more expensive. It consists in building a large chamber or cistern in the bottom of the outlet ditch, which is covered over by the cast-house floor plates under ordinary conditions, and stands full of water all the time, the waste-water current simply passing across its top, but if a breakout occurs the iron will run into this cisterm and has to fill it up before it can go on through and make its way into the outlet sewer, the plugging of which constitutes a calamity of no mean order. If the cistern be made large enough it cannot be filled up with the iron from any one breakout and is safe from being overflowed.

It is frequently customary to have the ditch around the furnace interrupted by a solid structure of brick on which the iron trough beneath the tapping hole is supported, but I do not like this construction, as the heat is drawn through the hearth jacket by the poulticing action of this brick pier and the circulation of water is cut off by it. Moreover, in case of a breakout the presence of this dam in the ditch prevents the iron from draining away in that direction and may result in permanently filling the ditch with a mass of iron on one side of the pier or the other.

By riveting a pocket a little larger than the outside of the iron trough air-tight on to the front of the hearth jacket the trough can be set down into this and a joint made between the two with cement, the water can then rise practically to the bottom of the iron trough without any danger of getting into it, and yet have an absolutely free circulation around the base of the hearth jacket at this point, which probably needs it more than any other.

In very recent years the security against breakouts has became so great that in some cases, the ditch is abolished and the hearth jacket is concreted in solid.