In searching for ironstone, it is frequently found ex posed to view in the broken ground ; but if this is not the case, numerous small pits must be sunk along the district under survey, and a series of boles put down if necessary. Few fields of ironstone have been fitted to a great depth, and drained with machinery. The great er part of the ironstone mines in Great Britain are either wrought level free, or are drained and V, r,:tight in con nection with coal mines in the same stratification.
Having thus described the position of coal in a geo logical point of view, particularly its relation to the other strata composing the upper part of the globe ; and having also stated the kinds of alluvial cover which rest upon it, the dislocations incident to the strata, the changes in the beds of coal termed troubles, the mode of survey ing a country for coal, the process of boring for it, and asertaining the different strata, the next department of mining to be described is, the manner pursued to ren der a coal workable ; termed the fitting or winning of a coal.
If a coal, or a number of coals, are found to exist in a coal-field, the first obstacle which prevents a pit's be ing sunk to any considerable depth is water, which is generally found in great quantity, particularly in the first opening of a coal-field, (if the pit is sunk near the crop,) which water, whether found in pits of very mo derate or very great depths, proceeds from the surface of the earth, though a contrary opinion was once held ; it being evident and certain, that the source of all wa ter found in mines is rain, which percolates through the alluvial cover, excepting in those cases when coals are wrought under the waters of the ocean ; for, it was shewn in the vertical section of the strata, that every stratum, however deep it may be from the surface in any one place of a coal-field, always rises till it meets the alluvial cover, comes to day, or crops out, as it is termed, excepting it is met by a slip or dislocation of the strata. if the crops of the strata are covered with alluvial matter containing water, such as gravel, quick sand, or any kind of soil pervious to water, this water will percolate through the pores, fissures, and beds of the strata, and force its way into that place of the pit or mine where the lateral resistance is taken away. If the strata crop out into beds of rivers, this circumstance produces, in general, an uncommon growth of water in the coal and adjoining strata.
From the circumstance of the water being so abundant in coal-fields, and presenting such an obstacle to the operations of the miner, coal-fields are divided into two kinds, viz.
1. Level free coal.
2. Coal not level free.
The level of the surface of the ocean being the low est point for drainage upon the surface of the globe, all coals and strata situated at a higher level than this surface are physically level free ; and all coals and stra ta situated under that line are physically not level free.
Therefore, in a general point of view, the surface of the ocean is strictly the line which divides these portions of coal-fields which are level free, from those which are not level free.
Though this definition is true in a philosophical point of view, many coal-fields, though level free, are situated at such a distance from the shores of the ocean, or arms of the sea, that the expence of bringing up a level or mine would be so great as to render such an operation out of the question ; and therefore, in the practice of mining, if a coal-field, or portion of it, is so situated above the surface of the ocean, that a level can be car ried either from that point, or from any hollow ground inland, till it intersects the coal, such field of coal, situ ated above the point of intersection, is termed a level free coal ; wheteas if a coalfield, though situated above the level of the ocean, cannot, on account of the ex pence, be drained by a level or mine, but by machinery, such coal-field, or portion of a coalfield, so drained, is, in the miner's language, termed not level free, though it is physically so.
From these principles, it is obvious that all coals and minerals situated under the level of the ocean must of necessity be drained by machinery ; and that all coals situated above that level may be either wrought level free, or by machinery, as may be thought most expe dient. This expediency is a matter of calculation ; viz. whether, in draining a given area of coal-field, the esti mated expence of driving the level, and upholding the same in repair, is less titan the estimated expence of erecting an engine with all its appendages, taking into calculation the annual expence of working and uphold ing the same ; to which must also be added, -a sum for contingencies. It must also be considered, that the le vel is constantly effectual in its operation, and is not af fected by additional feeders of water ; whereas each ad ditional feeder of water to an engine always produces an additional expcnce ; besides, the engine wears out, and must be renewed, or, if sold at any time, does not in ge neral bring one-third of the first cost. In these calcula tions must be considered the time required for complet ing the winning in both ways ; and if the time requir ed for executing the level greatly exceeds that for erect ing the engine, this circumstance alone may, in some cases, produce a determination to erect an engine in preference to making a level, though the latter was found in calculation to be absolutely less. There are instances, however, where, in order to save time, it may be expe dient to make a partial winning, of little depth, either by a day-level, or by machinery ; and while the coal laid dry by this winning is working, a deep day-level is progressively bringing forward, which, as soon as it in tersects the coal, and communicates with the engine level, supersedes the use of the engine.