The term burnt coal is riot known amongst miner alogists, or rather is not admitted ; the name is applied by the unlettered miner, from the simple ideas rising in his own mind, being altogether a stranger to those theories which so warmly interest and so much divide the philosophic world. It must be particularly remark ed here, that when a coal-field is working of the com mon coal, it is sometimes gradually changed into a species of glance-coal, which burns of itself, when ig nited, and is used for economical purposes, has no smoke, and only a blue flame, with intense heat. This is not reckoned one of the species of the burnt coal of the mi ner, but is simply one of those changes in quality inci dent to coal.
11. Glazed Backs, Lips, or Leips.—This is a trouble in the coal more generally found in an oblique direc tion as to roof and pavement, seldom perpendicular ; they are also more frequently found lying in the line of bearing, than in a crop and dip direction, and are distinctly different from the natural divisions found in coal. Although there can scarcely be perceived any slip in the roof, it is evidently a slip of the very least degree which occasions this trouble. Both sides of the fissure are smooth and glossy, and the coals thus divided have no tenacity, which renders this trouble dangerous to the miner by the coal falling suddenly forward without warning. A single fissure of this kind can scarcely be called a trouble, but when many of these are near to one another, the coal is injured, and the roof rendered very dangerous.
12. Troubles in the roof and pavement of the Coal.— These troubles are more generally found when ap proaching a dislocation of the strata, though they are occasionally luund in other parts of the coal-field. They affect the roof with narrow pendent ridges of stone or protuberances, at times very close together, in other instances like long inverted waves. The troubles in the pavement are generally sudden wave like swellings. These troubles frequently affect the coal very much, rendering it sometimes not only firmer in the texture, but uncommonly hard ; at other times the coal adjoining them is soft, and so deteriorated as to be altogether useless.
Besides these troubles, there is frequently another kind found in the roof, of a very singular form, known by the name of a pot bottom, or cauldron bottom ; they arc from the size of a few inches to five feet in diame ter. Ode of these is represented in Plate CCCLXXXIX. Fig. 5. In working the coal, the miner generally knows that he is approaching one of these by the coal becoming harder and more twisted in its texture, and this continues till the trouble in the roof is passed. The general form is similar to that represented in Fig. 5. a is the bed of coal, b the pot bottom, having an ir regular and rough inverted month of stone, d is coal generally from half an inch to an inch thick, altogether different in texture and appearance from the bed of coal, with which it is connected, being of a bright pitchy lus tre, and breaks into very small pieces ; sometimes it is of the nature of glance coal. The stone b which fills
the inverted pot is frequently of the same kind of stone which composes the roof, but more frequently is an ar gillaceous stone, of the nature of good fire-clay. The sides of the pot bottom at d are generally as smooth as glass and furrowed in the vertical direction, so that there is no tenacity where the sides of the pot bottom join the roof ; this circumstance renders these troubles very dan gerous, particularly when of a large size. The pecu liar singularity attending this trouble, is the uniform twisted texture and alteration which are found in the coal immediately under it, without any mixture of the stone which composes the pot bottom.
The Roofs of coal are formed of all varieties of the coal strata, as may be seen from the examples of strati fication in the preceding part of this treatise. As roofs they are of various quality, from good to very bad; a roof is reckoned good which keeps entire, not only while the coal is tvorking, but for a considerable time after. Sometimes a stratum of roof stone next the coal comes down along with the coal while working, named a following, leaving a firm roof-stone above ; at other times a stratum or two of the roof-stone have to be taken down shortly after the coal is wrought : this is also termed a following ; and while the miner works under it, he supports it by prop-wood for his own safety ; but these arc not reckoned dangerous roofs.
A bad roof is that, which, having little tenacity in itself, cannot be kept up either in boards, rooms, or in narrow mines, even by the aid of prop-wood ; it has, therefore, to fall in the waste, care being taken to keep it up along the wall-faces; and the iron rail-roads are in some bad roofs laid up .n the top of the fallen strata. The roofs which are most dangerous to the workman are those where the roof stratum is thick, with many open cutters in it, or where there are irregular beds, thick in the middle and wedge-shaped towards the edges. These roofs give no warning before they fall, by which the danger is much increased. The ordinary way in which a miner tries the roof to be safe or not, is to strike it gently with the side of his pick ; if the sound produced is sharp and clear, he concludes that the roof is good, but if the sound is obtuse and hollow, he is certain that the roof is bad and dangerous. He may, however, be deceived in trying a roof composed of a thick stratum. as it will produce a clear sound, though ready to fall, owing to the cutters with which it is inter sected.