Besides the large blast-furnaces, there are a great number of smaller air-furnaces, in which the old iron is re-melted. These are constructed in the form of great chests, but contracted towards the middle, where the iron is put, and then widening out again to the chimney, which is built at a great height, for the sake of the draught. At the part of the furnace which is farthest from the chimney, the coals are put through a small door; and the air forcing its way through the fire, to get up the chimney, is contracted into a focus upon the narrower part of the furnace where the iron is put, and effects a compleat fusion in about three hours. These furnaces are built with fire-bricks, with a bed of sand for the melted metal; and in this operation the coals are put in as they come out of the pit. In these stupendous works we saw models of some iron bridges which had been already made, and parts of others now manufacturing.
The first iron bridge, upon the principle of the celebrated Thomas Paine, (the inventor) was cast at Messrs. Walker's works, under his own inspection; but although it answered to a certain degree, it was deficient in strength. This defect has been since remedied in the iron bridges that have been cast here, of which that at Sunderland is the most famous; and there is one now in hand for the river