If coals have been wrought, or are working in the district of country under survey, particular inquiry must, in the first place, be made, as to the quality and thick ness of the coals, their general dip, lire of bearing, and shape of the coal-field, also if any dikes or slips are known to exist, and if they do, to learn their line of bear ing, and extent of the dislocations of the strata produced. The form of the coal-field is important, as, without a knowledge of this, much difficulty and great mistakes may be the consequence. When numerous collieries are opened in a district of country, the subterranean ge ography of the strata is well known, the series or sys tem of beds of coal composing the coal-field are ascer tained, with their accompanying strata ; and to an expe rienced miner they are as well known as the leaves of a book, excepting in cases where great derangement and dislocation of the strata have taken place. It is also of importance to know if the coals are liable to such trou bles as alter their quality, render them thinner, or if they are separated occasionally by bands of stone, which in some instances separate the upper and lower parts of the bed of coal to such an extent as to form two coals.
The same general survey and investigation of the dis trict must be followed according to the rules formerly laid down ; and if no distinct view of the coals or strata can be seen, so as to draw accurate conclusions, the far ther investigation must proceed by boring, care being taken not to overstep the coal-field, or beds of coals which are known to exist. This mistake will take place with the most cautious, if the form of the coal-field is not known and attended to.
The art of boring for coal is one of the chief points in mining, in order to ascertain the strata and beds of coal, before any great operation is commenced in win ning or establishing a colliery, in which a great capital is frequently invested.
The rods commonly used are made of the best Swedish iron, about an inch and a quarter square ; each rod is three feet in length, having a male screw at the one end, and a female screw at the other; these are what are named the common rods. The chissels for boring are eighteen inches in length, and are from two inches and a half to three inches and a quarter in breadth at the cut ting edge, faced with the best steel; with these one short rod is connected, called the double box-rod, to which the chissel is screwed. It is also eighteen inches long, so that the chissel and box-rod together, form a three feet length equal to the common rods. There are also three short rods, named cut rods, of a foot, eighteen inches, and two feet in length, which are attached to the brace head, to render the height above the mouth of the bore suitable for the men to work the rods with effect ; by this arrangement, the whole sct of rods divides easily and accurately into yards and fathoms, for the more cor rectly keeping a journal of the strata passed through.
What is termed the brace-head rod is eighteen inches long, having two large eyes at the top, set at right an gles to each other, through which arms of wood are put, by which the men lift and turn the rods in boring. There are besides a number of other instruments connected with the boring rods, viz.
Wimbles, of various kinds.
Sludge rs.
Rounders.
These are occasionally attached to the rods, as after described. There are also keys of different kinds for supporting the rods, and unscrewing them, when draw ing them up, or letting them down into the bore hole ; and when triangles arc used so as only to unscrew four or five fathoms of rods at a time, topits, or top-pieces are used for each length of rods so cut, and an instru ment, named the runner, is used for taking hold of the topits. Besides these, there are several instruments named beches, with contrivances for catching hold of the rods when they break down in the bore, which is not an unfrequent occurrence. When bores are only to be a few fathoms in depth, the whole operation is performed by manual strength ; but when a deep bore of any con sequence is to be made, a set of lofty triangles of wood is placed over the bore-hole, with a pulley at top, through which a rope is passed ; one end is connected with a crane or windlass at the outface, to the other end, an oval iron ring, named a runner, is attached ; by these means the rods are drawn up and lowered down with great facility.
In England, particularly in the Newcastle district, there are professional master-borers, who undertake to put down bores for coal, and give a precise journal of the strata passed through. The average price of bor ing in England and Scotland, where no uncommon dif ficulties occur, is six shillings for the first five fathoms, and an increase of six shillings each fathom for every additional five fathoms, viz.
In this manner the price increases regularly with the depth ; and in all common cases the master-borer finds and upholds the boring rods, but there are instances where the price is considerably higher than the above rate.
In Scotland, boring for coal is scarcely known as a profession, but there are master sinkers who occasion ally bore, and who perform the operation with accuracy.