reduced so much that it cannot be carried further if additional lining-tubes are introduced, and it becomes desirable to drive the last set of lining-tubes further down the hole. To make this possible, it is necessary to rime out the hole. Numerous kinds of rimers have been made, some consisting of curved chisels with sharp sides opening by means of a spring.

The kind of rimer made by Kind is shown in Fig. 108. In this case the cutter is made of two pieces of steel hinged on the pin b, with shoulders, a, on which

is formed the cutting edge. The cutters can be closed together as shown in Fig. 108, or they can be opened out as shown in Fig. 109. This is effected by means of the wedge c, which is drawn up by the cord d, opening out the chisels to the distance permitted by the link h. By means of this or similar tools the hole may be enlarged.

It is evident that this form of chisel leaves a shoulder of rock, e, supporting the tubes. In order to remove this shoulder, a similar tool to that shown in the figure is used. But the cutting edge b is formed to cut upwards, as shown by the dotted lines in Fig. 109, so that by jerking it upwards the shoulder e is cut away.

Diamond Drill. The diamond drill is an example of the process of boring by grinding instead of percussion. The diamond is the hardest or one of the hardest substances known, and is much harder than any rocks through which borings have to be made, and therefore, if a diamond is pressed against any other substance and then moved along the surface of this substance, it will scratch it. There is a species of diamond which is of no value for ornamental purposes, and is therefore much cheaper than the brilliant; it is this dull kind of diamond which is used for boring. Fig. 110 shows the apparatus. Fig. 111 is the boring head or crown, which may vary from 2 inches up to 18 inches in diameter. This is a ring of steel, with shallow holes dovetailed as shown by the black marks; into these holes the diamonds are placed surrounded by some soft metal, and then the metal is hammered over them, completely enclosing the

stone, all excepting one slightly projecting point or corner. This boring-crown is screwed into the end of a long length of

steel tube (Fig. 112). At the surface the tubes are gripped in a tube, Q Q (Fig. 110), which has on the outside a rib or feather; this feather slides vertically in a collar, which is fastened to and made to revolve by the bevelled gearing B. The upper part of the boring-tube is connected by water-tight and swivel joints U with a hose-pipe connected with a forcepump, P; by means of this pump water is forced into the interior of the tube, and down to the bottom of the hole, where it issues underneath the boringcrown. When the engine is started, the boring-rods revolve at a speed of perhaps 200 or 300 revolutions a minute, and as they are a great weight they force the diamonds against the rocks, which are abraded; at the same time the water, issuing from the internal tube, carries away the sand that is made, keeping a clean

SECTION

PLAN

FIG. 111.-Bor-
ing-crown.

b.....

b

a.

FIG. 112.-Core-extractor.

The tubes are free

surface against which the diamonds can act. to slip down inside the outer collar, and thus are continually pressing at the bottom of the hole as it becomes deeper. When the tubes have fallen the length of the outer collar, the grippingtube is unclamped from the boring-rods and raised up to the top of the outer collar, when it is again clamped, and fresh rods are added at the top as the bore-hole gets deeper. As soon as the borehole gets a considerable depth, the weight of the rods becomes too great to rest upon the diamonds without causing too much friction; this weight is reduced by balance-weights. Wire ropes or chains from the balance-weights pass over pulleys R, and are connected with a loose collar, H, fixed on the gripping-tube, within which it is free to revolve, but by which the weight of the rods is supported. The deeper the hole, the greater the balance-weight required.

Cores. As the boring-crown only cuts a ring, the centre part remains and forms a core, which passes up inside the hollow rods. The lower part of the rods are called a core-tube, and this tube is sometimes of sufficient length to admit a core of 20 feet, and is usually of larger diameter than the rods above; but in order that the core may be withdrawn at the same time with the rods, it is necessary that there should be some kind of grip

or grapnel inside the core-tube. This is made in the following manner: Three vertical grooves at opposite sides of the coretube are cut with an incline so that the depth of the groove is less at the bottom of it than at the top, a a (Fig. 112). Into each of these grooves is placed a sliding block of steel, bb, with serrated edges inside; these blocks press against the core, and are lifted

FIG. 113.-Diamond boring-drill.

up by it, as it enters, towards the top of the groove, where, owing to the greater depth of the groove, there is room for the core to pass. If now the core-tube is raised, and the movable blocks, catching against the core, remain stationary, the inclined surface of the groove will press them so tightly against the core, that by the time the bottom of the groove comes against them and forces

them up, they will also break off the core from the bottom, and bring it up inside the tube (a split ring is sometimes used instead of separate pieces).

This method of boring is particularly applicable to hard rocks, and is nowadays generally employed in such ground. In rather tender ground, such as is common in the coal measures and in the New Red Sandstone formation, borings have not always been successful, because the core has been shattered by the shaking of the drill, and then carried away in the form of sludge; the nature of the sludge has escaped detection, and a bore-hole has been known to pass through a good seam of coal, unknown to those in charge. In order to get good cores in the coal measures, the hole should not be less than 6 inches in diameter, and must therefore be started at a still larger diameter. A special core-holder is made to protect the core, and has been used with good results.

FIG. 114.-Boring head. FIG. 115.-Core bit. FIG. 116.-Safety-clamp. FIG. 117.-Liftingjack. FIG. 118.-Bayonet clutch coupling. FIG. 119.-Core-lifter.

This core-holder has an internal tube round which the boring crown revolves, the internal tube remaining stationary; the water from the boring-rods passes down between the internal and the external tubes; the core is thus saved from direct contact with the revolving tube and the stream of water. At the top of the internal tube are some holes through which the water escapes as the core rises up.

A similar machine (see Fig. 113) is used in the ironstonemines of Michigan, U.S.A., for prospecting. This machine is taken into the mines, and bores a hole through the rock at any angle that may be desired, vertical or inclined, either at an angle of 45° or level. The head A (Fig. 113) revolves, and can be clamped at the desired angle. On the machine is a winding-drum, B, by which the rods can be raised or lowered. Fig. 114 is a boring-crown, which grinds all the stone away. Fig. 115 is a core