When the pillars of a panel are to be worked out, the most distant range is first attacked, and as the workmen cut away the furthest pillars, props of wood are placed between the pavement and the roof, within a few feet of each other, as shown by the dots at I. This is continued till an area of one hundred square yards is cleared of pillars. This operation is termed ‘working the _goaf_.’ The only use of the prop-wood is to prevent the stratum which forms the ceiling over the workmen’s heads from falling down and killing them by its splintery fragments. Experience has proved that before proceeding to take away another set of pillars, it is necessary to allow the last made goaf to fall. The workmen then begin to draw out the props. This is a most hazardous employment. Knocking down the more remote props one after another, they quickly retreat under the protection of the remaining props. Meanwhile the roof stratum begins to break by the sides of the pillars and falls down in immense pieces; while the workmen still persevere, boldly drawing and retreating till every prop is removed. Nay, should any props be so firmly fixed by the top pressure that they will not give way to the blows of heavy mauls, they are cut through with axes, the workmen making it a point of honour to leave not a single prop in the goaf. If any props are left, it causes an irregular subsidence of the strata, and throws more pressure on the adjacent pillars. The miners next proceed to cut away the pillars nearest to the sides of the goaf, setting prop- wood, then drawing it, and retreating as before, until every panel is removed excepting small portions of pillars which require to be left under dangerous stones, to protect the retreat of the workmen. While this operation is going forward and the goaf extending, the superincumbent strata, being exposed without support over a larger area, break progressively higher up; and when strong beds of sandstone are thus giving way, the noise of the rending rocks is very peculiar and terrific; at one time loud and sharp, at another hollow and deep. As the pillars of the panels are taken away, the panel walls are also worked progressively backwards to the pit-bottom, so that only a small portion of the coal is eventually lost. When mines are fully worked, the main shaft is frequently continued down to other seams of coal, which are excavated in the same way as the first seam. In such cases the workings communicate with each other by shafts called ‘staples,’ which are sunk at intervals between the seams of coal. [Illustration: GENERAL VIEW OF MINING OPERATIONS.] The principle of long-wall working (Shropshire and Derbyshire method) is the extraction of all the available coal by the single process of first working, maintaining the roads by means of stone-walls or wooden props. This system is applicable with advantage only to thin seams which lie near to the surface, and in which the workings may be of a very limited extent. [Illustration: