If a line of bores is to he conducted from the dip of the strata towards the rise or direction of the crop, the same rules hold as to the placing the bores at the proper distances from each other, according to the dip or the angle of inclination made by the strata with the horizon; with this correction, that the bores are placed some yards nearer each other than the distance resulting by multiplying the dip by the depth of the bore, in order not to overstep the last strata passed through in the pre ceding bore ; for if any of the strata are overstepped, several of them will not appear in the journal, and a va luable coal may be in these strata which have escaped being bored through. The general rule in this case is to place each successive bore so that the first of the strata passed through shall be the last which were passed in the bore immediately preceding. The bores No. 4, 3, and 2, elucidate this principle in boring from the dip to the crop ; but suppose that the bore No. 2. had only gone to the depth of the stratum f, and that the bore No. I. was put down in the position as marked in the Figure, in this case the stratum e, with the coal immediately below it, would be overstepped, none of the bores would pass through them; of course they would not appear in the journal, and the existence of that coal would be unknown.
Having described the mode presently practised in boring in the crop and dip-line of direction, where the dip is known before the boring is commenced, the next operation to be described is the proof bores in the line of bearing, which are necessary in proving a coal-field in a district where coals were not known to exist before. The line of bearing being at right angles to the line of dip and crop, let it be supposed that these bores are to be put down at right angles to the bore No. 4. at the distance of four to six hundred yards ; the one on the west, the other on the east side. (Plate CCCXC. 2.) After making the requisite allowance for the differ ence of level betwixt the top of these bores and No. 4. bore, the two coals found in the bottom of No. 4. ought to be found at the same depth in each of the bores put down in the line of bearing if the strata are lying regu lar and free from dislocations ; having, however this cor rection in view, that as all coal fields are bason-shaped, and of very various magnitudes, the absolute line of level along any bed of strata deflects either inwards or outwards from the line of bearing, which is at right an gles to the line of dip and crop ; and, therefore, refer ring to the shape of coal-fields formerly discussed, it will be seen that bores put down in a proper boson formed field, in the line of bearing, will strike the coals sought after sooner than calculated upon; and, in the case of the form being an inverted boson, the coals will will be deeper than calculated; in large bacons this dif ference in the line of bearing will not be much when at a moderate distance from the dip-bore, but in a small bo son the difference will be found considerable, for this very evident cause, that if the boson is not broken by disloca tions of the strata, all the coals crop or come to day in the line of bearing. Having these corrections in view, if the coals are found at a depth corresponding to the calculations made from practical data, it may be con cluded that the coals are lying in that field, so far as proved, fair and regular ; but if these bores require to be put down to a much greater depth than was calcu lated upon belore striking the coals found in the bottom of No. 4. ; or if these coals are found much sooner than was expected, it may be then concluded that a slip or dislocation of the strata exists betwixt the two bores, throwing down the strata in the first instance, or throw ing them up in the second. Under such circumstances a few additional bores will be requited, to ascertain the coal-field still farther, and the line or direction of the slip, particularly if the dislocation is of considerable magnitude. The position of such bores can only be de termined on by weighing every circumstance which may occur while the operation of boring is carry lug for ward.
If in proceeding with a series of bores where the line of dip is not known, nor consequently the line of bear ing, it is an important anti primary point to ascertain this,not only as regarding the situation of the coalfield, but how it ranges within an estate or particular property. The mode practised is by putting down three bores in the following position. Suppose a bore No. I. (Fig. 3. Plate CCCXC ) put down, which reaches a coal at the depth of 50 at the distance of 300 yards a place is marked for No. 2. bore at B, and a place for No. 3. bore at C, equidistant from No. 1. and No. 2., so that the bores are placed at the angles of an equilateral triangle ; then suppose the bores No. 2. and 3. to be put down, and that the coal is found in No. 2. at the depth of 30 yards, and in No. 3. at the depth of 44 yards ; from this it is evident that neither of the lines AB, BC, or CA, are in the line of level, which in short distances, as be fore mentioned, approximates to the line of bearing, particularly in coals which have a very moderate dip. As No. I. is the deepest of the three bores, and No. 3. the next in depth, this shows that the line AC is nearer the line of level, than either the lines AB or BC. The question therefore is, at what distance upon the protract ed line BC, is the point at which, if a bore were put down, it would reach the coal at the same depth as at Nu. I. viz. at 50 yards. It is thus resolved : as 14 yards the difference of depth betwixt the bores No. 2. and No. 3. is to 300 yards the distance betwixt them, so is 20 the difference of depth betwixt No. 1. and No. 2. to the proportion resulting, which is 428 yards, I foot, 8 inches ; this distance, measured from No. 2. reaches to the point D on the protracted line BC, at which point D, the coal will be found at 50 yards deep, as at A; hence the line AD is the true level line of the coal or strata. A line drawn at right angles to All, such as BF, is the true dip line of the coal. In this example, the dip, in the miner's language, is 1 in 143, that is, in each distance of 143 yards, measured along the line from B to F, the coal is found one yard deeper. Hence a calculation is easily made at what depth the coal would be found at any point of distance from B, along the pro tracted line BG, viz. by dividing the given length by the dip. If the distance from B to G in the line of dip is 455 yards, this, divided by 143 the dip, gives 30 yards, which, added to 30 yards, the depth of No. 1. gives 60 yards for the depth at which the coal would be found at G. As any line drawn at right angles to the line of level AD, is the line of dip, so any line drawn parallel to All is a level line. Hence, if from No. 3. bore, a line CE is drawn parallel to AD, the coal at the point E will be found upon a level with the coal at C, viz. at 44 yards. The point E may, however, be found on a level with No. 3. by a rule of proportion similar to the former, viz. as 20 yards the difference of depth betwixt No. 1. and No. 2. so is 300 yards the distance betwixt them, to 14 yards the difference of depth betwixt No. 2. and Nc. 3. the proportion resulting is 210 yards; which distance, measured from B, is found to be at E. Had this pro portion been first tried, then A D drawn parallel to EC would have given the point D, upon the protracted line BC, where the coal would be found on a level with the coal at the point A. In this example, the surface is sup posed to be level ; but as this is seldom the case, the re lative situation of the bores at the surface with each other must be accurately ascertained. If No. 1 is situ ated higher than either No. 2. or No. 3. the difference of altitude must be added to each bore, so as they may be equal to No. I. which correction brings them to the same horizontal plane, and then the projection and cal culations are made as before exemplified. It is by this theorem for finding the line and dip of level, that the most eligible spot in a coal-field or coal propel ty can be selected ; where, by sinking a pit, the coal, under all cir cumstances, can be wrought to the greatest advantage, particularly as to the drainage of the coal-field.