When a geological examination of the surface shows the existence of carboniferous rocks, a further search may be made to ascertain the possible occurrence of a coal-scam or seams. This is best done by making a detailed examination of the exposed face of any cliff, cutting, or quarry, and searching along the valleys of all rivers or streams passing through the carboniferous area. In this way an exposure or outcrop of a seam may be discovered. In such a case a little digging will penetrate the weathered surface and expose the seam, so that its thickness and quality can be obtained as well as the nature of the underlying bed or 'floor' and the overlying bed or 'roof.' The nature of the rocks forming the floor and roof of the seam have great influence on the operations of mining, and information regarding them is of much value. The direction and amount of dip should also be noted. Having located a seam in this way, its outcrop may be traced over a considerable distance, and some opinion formed as to the possibility of working it profitably. If conditions appear favourable, the seam may then be opened by means of a mine from some suitable part of the surface, or by sinking shafts further to the dip, should this commend itself as the better course. While the above method of search may bear fruit in some cases, there are many others where the seams do not come to the surface, and in such cases, having first confirmed the presence of coal-bearing strata, it is usual to search for the coal-seams by boring.

Boring. — Two methods of boring are in general use: (1) The percussive method, and (2) the rotary method. In the percussive method a steel chisel attached to boring rods delivers a succession of blows on the bottom of the hole, the chisel being rotated through a small angle between each blow. The hole is deepened in this way, and from time to time the rods and cutting tool are withdrawn and the debris raised by means of a sludge-pump. Where the hole passes through soft rocks the sides are liable to fall in, and to prevent this, the hole is lined with light steel or wrought-iron tubes. This system is subject to two disadvantages The first is that it can only be used in cases where the hole does not require to penetrate to a depth much beyond 1000 feet. The second and most important objection relates to the difficulty of obtaining accurate and trustworthy information about the nature and thickness of the beds passed through. This information has to be obtained from a careful examination of the debris, and it is frequently so mixed with material from the sides of the hole and from immediately overlying beds as to render the getting of reliable results extremely difficult. This disadvantage increases with the depth of the hole, and for this reason the percussive system is only used for shallow holes, when definite information is wanted. On the other hand, if boring for oil or water, where it is only a question of getting the hole down to the oil or water-bearing bed, and where no definite information regarding the nature or thickness of the intervening beds is required, the percussive system is extensively used, even for comparatively deep holes.

Where accurate information regarding the nature and thickness of the beds penetrated is desired, it is better to use the rotary method of boring. In this method the rods are formed by a series of screw-jointed tubes, at the lower end of which the cutting-head, or crown, is fixed. This cutting-head is cylindrical in shape, and hollow like the rods, to which it is attached by screwing. On its lower face it carries a number of diamonds, which are set into the metal, and so arranged that they project beyond both the outside and inside edges of the crown. The crown and the tubes connected to it are rotated by a suitable mechanism, and the diamonds fixed in the crown cut an annular space in the rock leaving a solid core inside. This core is broken off at convenient intervals by a suitable appliance, and brought to the surface by the tubes. A stream of water is forced down the tubes during their rotation, and passes out under the crown, ascending between the sides of the hole and the outer surface of the tubes, carrying the fine debris cut by the diamonds with it.

The cores, after being brought to the surface, are usually placed on a floor, in the order in which they are taken from the borehole, and thus exact information regarding the nature of the rocks passed through, the thickness, and relative position of the various beds, is obtained. The general arrangement of a typical diamond drill is shown in Fig. 29.

Smaller drills, suitable for use underground, or for holes up to 300 feet deep on the surface, are also made. These small machines are usually driven by manual power, and are arranged to bore either vertically or any angle that may be desired.

Black Diamonds for Drills. — Carbon or black diamond is the hardest substance known, being even harder than the gemstone. Although called black, its colour ranges from steel grey to dark brown. It is amorphous, not crystalline, like the true diamond, and is found only in two small districts in Brazil, where it occurs as water-worn pebbles in a conglomerate. The stones range in size from small fragments to pieces weighing two or three hundred carats. Small pieces weighing from one to four carats are used for the drill crowns, about eight pieces being used for each crown. These pieces are inserted in holes bored in the metal, and held in position by caulking the metal back over the body of the stone, leaving only a small projecting part standing about one thirty-second of an inch above the surface of the metal. A crown with the diamonds set ready for use is shown in Fig. 30.

Depth and Cost. — Holes have been bored by diamond drills to a depth of nearly 7000 feet, the cost of these deep holes being proportionately greater than shallow ones. Compared with the percussive system, diamond boring is more costly, but it has a greater range of depth, and furnishes such exact information that it is the best system to use when prospecting for coal.

Dip of Strata from Boreholes. — If the coal is found at a moderate depth from the surface, it is usual to put down several boreholes at various parts of the field. When the coal lies at a great depth it would be too costly to do this, and fewer boreholes are made to supply the requisite information. Where three boreholes not in the same straight line are put down, the dip can be ascertained thus. First, the depth of each borehole is measured from the same horizontal plane on the surface; if the surface is irregular or inclined it must be levelled by means of a levelling instrument, and the depth of each hole calculated from the same datum. Second, the distance between each hole is measured and the position of each hole marked on a plan drawn to some suitable scale. A point is then found on one of the lines joining two of the holes at which the seam is the same depth as it is at the remaining hole. A line drawn from this point to the remaining hole gives the direction of strike or level course, and any line at right angles to the line of strike gives the direction of dip and rise. That this is an easy and interesting construction may be seen from the following example. A coal-seam has been proved by boreholes at X, Y, and Z. That at X is 300 yards deep, that at Y is 350 yards deep, and that at Z 400 yards deep. The distance from X to Y is 800 yards, and from Z to X 900 yards, while from Y to Z it is 1100 yards (Fig. 31). Construction. — Draw the triangle XYZ to some suitable scale, then find a point between X and Z, at which the seam is at an equal depth to that at Y. Thus X is 300 yards deep and Z is 400 yards deep, hence the seam dips 100 yards in the 900 yards from X to Z. The dip from X to Y is 50 yards, the difference of depth of the two holes, so that a point can now be found on XZ at which the seam is 50 yards below Z, by proportion

900 x 50

100

= 450 yards from X. If this point, which we will call M, is joined up to the point Y, the line YM will represent the line of strike, and a perpendicular to YM will give the direction and amount of dip of the seam. In this case the line XN drawn through X and perpendicular to YM is 420 units, measured to the same scale as the sides of the triangle XYZ, and the amount of dip is

50	=	50
XN		420

which is equal to 1 in 8.4.

If the magnetic bearing of either of the lines XZ or YZ be determined, the direction of the dip can be found from the construction by aid of a protractor.

In deep holes made with a diamond drill, the core is marked and the direction of dip ascertained by comparing the direction of the dip shown by the section and the fixed mark upon it.

Shaft Sinking. — If after boring, the information is of such a character as to make it appear that coal exists under conditions that will admit of profitable working, then shafts have to be sunk to reach the coal. The position of the shafts depends upon a variety of circumstances, but usually ease of transit of the coal by rail or water is the most important. In order to comply with the Mines Act two shafts must be provided. These shafts must be at least 15 yards apart, with an opening not less than 4 feet by 4 feet connecting them. One of these shafts will carry the air current down into the workings, the other provides a way for the air, after having ventilated the mine, reaching the surface. The shaft through which the air enters the mine is called the downcast, that by which it leaves the mine is called the upcast. Both shafts must be fitted with winding gear whether they are used for purposes of traffic or not. The object of having two shafts is to provide suitable means of ventilation and ensure the safety of those underground in the event of one of the shafts becoming blocked. The size of the shaft depends on the output of coal to be drawn per day, and upon the depth from which the coal has to be raised. The output per day depends upon the quantity of coal contained in the area under lease, and the length of time granted for its removal. The shape of the shaft may be square, rectangular, circular, or elliptical. In practice either the rectangular or the circular forms are those generally adopted.

The rectangular shaft must be secured by a timber lining on account of its shape, and since timber has a limited life (from fifteen to twenty years when used as shaft-lining), this form of shaft is often adopted when its period of use is not likely to extend much beyond twenty years. The circular form of shaft is secured by a lining of brickwork, and is the form usually adopted for deep shafts that are likely to be used for longer periods. Custom and experience of sinking particular forms are sometimes largely responsible for the shape adopted. In Scotland the rectangular form finds most favour, while in England the circular form is generally preferred. When a shaft has to be sunk through soft mud or gravel the circular form is almost invariably adopted, as it is the only one that allows of suitable methods being used to penetrate soft or water bearing beds.

Sinking a Rectangular Shaft. — The first thing in starting a shaft of this kind is to get it placed in its correct position with regard to the dip and rise of the coal-seam. The common practice is to place the long sides of the rectangle parallel to the dip and rise; the openings are then facing the strike or level course, the usual direction from which the coal arrives at the pit bottom. Once the desired direction of the sides has been ascertained the ends must be set out at right angles to the sides, everything being laid off to the dimensions previously fixed. With this done, excavation is commenced, and carried down through the surface to a depth of several feet, and the sides supported by temporary wood until a convenient depth or the rock head is reached, when the permanent lining, or barring, as it is usually called, is put in. This barring is usually of pine, 9 inches broad, by 3 inches thick, or more if required, and of such length as will provide the size of shaft already agreed on. A bed is squared and levelled, and the first set consisting of two side and two end bars are then placed in position, being wedged behind to prevent them from moving.

After this, set after set is built in on the top of each other until the surface is reached, plumb-lines suspended from a frame at the top being used to keep the work perpendicular. Further support is given to the shaft by cross timbers, called buntons; these are driven down on pieces of timber, usually 11 inches broad by 3 inches thick and 18 feet long, called wall-plates. The fixing of the cross timbers is usually kept behind the last 15 or 20 feet of barring to prevent them being knocked out while blasting. A filling-in piece is used at each of the corners called a corner racking. A plan and section through the centre of a rectangular shaft is shown in Fig. 32.

As soon as the lining of the section is completed, sinking is resumed and the shaft carried down for a further distance. A new section of lining is now commenced, and carried up until it closes with the section above, and step by step the shaft is carried down until it reaches the coal. Generally the material through which the shaft passes has to be blasted out by explosives, and after excavation it is raised to the surface by means of a bucket or kettle. Until lately it was a common practice to run the kettle free, but now, where the depth of the shaft is over 100 yards, it must be run in guides provided for the purpose. Various arrangements for discharging the contents of the kettle when it reaches the surface are adopted, the selection depending on the size of the shaft and amount of debris to be handled.