In South Wales and other mountainous districts it is the practice to build the blast furnaces beside cliff's or steep precipices, which are sometimes walled up artificially to render them perpendicular, so that the tunnel-heads of the furnaces shall be upon a level or somewhat lower plane than the contiguous high ground, from which a bridge is thrown on to the top of the solid masonry, after the manner expressed in the subjoined cut. This cut also gives the external form of the furnace described at page 758, with this difference merely, that instead of the open crater or tunnel-head, this has a vertical cylindrical chimney of the same internal area as the upper orifice built over it; on one side of which there is a doorway for charging the furnace with the mine and other materials. A parapet wall or iron fence surrounds the upper edge of the square tower. The dotted lines in the figure indicate the form of the interior of the furnace. The coke-hearths and the mine-kilns, and all other supplies to the furnace, are situated on the high ground, so that they can at once be shot across the bridge into the tunnel-head.

Various buildings, including the " cast-house," where the metal is run into pigs (or other forms to patterns as may be required) are situated on the lower level, and are covered with roofs.