The open-burning cubical coals are known by several local names ; the rough coal or clod coal, from the large masses in which they may be had, and the cherry coal, from the cheerful blaze with which they spontaneously burn ; whereas the caking coals, such as most of the New castle qualities, require to be frequently poked in the grate. Its specific gravity varies from 1.25 to 2. Slate or splint coal.—This is dull black, very compact, much harder, and more difficultly frangible than the pre ceding. It is readily fissile, like slate, but powerfully resists the dross fracture, which is conchoidal. Specitio gravity from 1.26 to 1.40. In working, it sepa rates in large quadrangular sharp-edged masses. It burns without caking, pro duces much flame and smoke, unless ju diciously supplied with air, and leaves frequently a considerable bulk of white ashes. It is the best fuel for distilleries and all large grates, as it makes an open fire, and does not clog up the bars with glassy scoria. I found good splint coal of the Glasgow field to have R specific gravity of 1.266, and to consist of—car bon, hydrogen, 4.3; oxygen, 24.8.
3. Cannel between velvet and grayish black ; lustre resinous ; fracture even ; fragments trapezoidal ; bard as splint coal; specific gravity 1.23 to 1.28. In working, it is detached in four-sided columnar masses, often breaks eon ehoidal, like pitch, kindles very readily, and burns with a bright white pro jeetive flame, like the wick of a can dle, whence its name. It occurs most abundantly in the coal-field of Wigan, in Lancashire, in a bed 4 feet thick • , and there is a good deal of it in the Clydes dale coal-field, of which it forms the low-. cst seam that is worked. It produces very little dust in the mine, and hardly soils the fingers with carbonaceous mat ter.- Cannel coal from Woodhall, near Glasgow, specific 1.228, consists by My analysis of—carbon, 72.22; hydro gem 3.03; oxygen, 21.05; with a little azOte (about 2•3.in 100 parts): This coal hits been found to 'afford, in the Scotch gas-works, 'a Very rich-burning gas. jThe ainte-is there converted into antalema, of which a considerable quantity is distilled over into the tar-pit.
4. Glance coa./...,-This speeies has - au iron-black color, with an occasional iridis eence, like that of tempered steel; lustre in general splendent, shining, and im perfect metallic ; does not soil ; easily frangible ; fracture flat conchoidal ; frag ments sharp-edged. It burns without flame or smell, except when it is BIli phnreous ; and it leaves a white-colored ash. It produces no soot, and seems, I indeed, to be merely carbon, or coal de prived of its volatile matter or bitumen, and converted into coke by subterranean calcination, frequently from contact with whin-dikes. Glance coal abounds in Ire- i land, under the name of Kilkenny coal; in Scotland it is called blind coal, from its burning without flame or smoke ; and in Wales, It is the malting or stone coal.
It contains from 90 to 97 per cent. of car bon. Specific gravity from 1•3 to 1.5; increasing with the proportion of earthy irnpurities.--(,Ure.) Dikes and faults are denominated up throw or downthrow, according to the position they are met with in working the mine. Thus, in figure (p. 466), if the miner in advancing to the rise, the dike A B obviously does not change the direction ; bnt o o is a downthrow dike of a certain number of fathoms towards the rise of the basin, and a r is an upthrow dike likewise towards the rise. On the other band, when the dikes are met with by the miner in working from the rise to the dip, the names of the above dikes would be reversed; for what is an up throw in the first case, becomes a down throw in the second, relative to the min ing operations.
We have seen that hitches are small and partial slips where the dislocation does not exceed the thickness of the coal seam ; and they are correctly enough called steps by the miner. This figure represents the operation of the hitches A, B, 0, D, E, F, 0, H, on the coal measures, though observed in one or two seams of a field, they may not appear in the rest, as is the case with dikes and faults.
The boring tools used for mining for coal are given below :— gt fitting or winning a coal-fteld.-1n sinking a shaft for working coal, tho great obstacle to be encountered is water, particularly in the first opening of a field, which proceeds from the surface of the adjacent country ; for every coal stratum, however deep it may lie in one part of the basin, always rises till it meets the alluvial cover, or crops out, unless it be met by a slip or dike. When the basset edge of the strata is covered with gravel or sand, any body or stream of water will readily percolate downwards through it, and fill up the porous interstices be tween the coal-measures, till arrested by the face of a slip which acts as a valve or flood-gate, and confines the wa ter to one compartment of the basin, which may, however, be of considerable area, and require a great power of drain age.
Tn reference to water, coal-fields are divided into two kinds :-1, level free coal ; 2, coal not level free. In the prac tice of mining, if a coal-field, or portion of it, is so situated above the surface of the ocean, that a level can be carried from that plane till it intersects the coal, all the coal above theplane of intersec tion is said to be level free ; but if a coal field, though placed above the surface of the ocean, cannot, on account of the ex pense, be drained by a level or gallery, but by mechanical power, such a coal field is said to be not level free.