J is a clamp by which the rope A is fixed while boring is going on.

Steam is admitted below the piston (Fig. 144) raising the pulley G; at the end of the stroke, the exhaust valve is opened, the steam escapes, and the piston, pulley, rope, and boring head all drop. The exhaust port is so arranged as to leave a cushion of steam which prevents the piston from striking the bottom of the cylinder. The steam and exhaust valves are worked automatically by tappets, M M, actuated by the piston-rod. The length of the stroke can be varied from 1 to 8 feet by shifting these tappets. The usual speed is 24 blows a minute.

The boring head (Fig. 145) forms a special feature of Mr. Mather's invention. The chisels or cutters D are fixed by nuts in the cast-iron block C; E is a cylindrical block serving as a guide, and F is a second or upper guide which assists in effecting the rotation. On its circumference there are ribs which catch in one direction; they are placed at an inclination, like segments of a screw thread of very long pitch. Each alternate plate has the projecting ribs inclined in the opposite direction, so that one-half of the bars turn the rod round in rising, and the other half turn it in the same direction during the descent; but they simply assist in producing the rotation which is mainly secured by the contrivance represented above F. Two cast-iron collars, G and H, are cottered to the top of the bar B, and their deep ratchet-teeth are set exactly in line with one another. J is a movable bush sliding upon the bar B, and attached to the boring rope by the bow K and a short piece of chain.

The bush J has ratchet-teeth on its upper and lower faces, but the upper teeth are set half a tooth in advance of the lower ones. During the ascent of the rope, the bush has the position shown in the figure; but when the tool strikes the blow, the bush descends, and the centre of the inclined surface of each lower tooth of J strikes the point of a tooth of G, and then slides down on it, twisting J, and with it the flat rope, to the extent of half a tooth. At the commencement of the lift the bush J receives a further twist of half a tooth by coming against H. The flat rope is thus twisted altogether to the extent of one tooth, and in untwisting it turns the tool a like amount; automatic rotation of the cutters is thus secured.

P (Fig. 143) is the shell-puinp, or sludger, and Q is an overhead suspension bar by means of which it is brought over the litt'e table R in the tank T. The screw S serves to raise the table R until the pump rests upon it, and on knocking out a cotter in the rod which supports the seating of the bottom valve, the sludge is speedily discharged.

One man can attend to all the operations of raising and lowering, changing the boring tool for the shell-pump or vice versâ,

K

and regulating the boring. Two labourers are employed to change the cutters and clear out the shell-pump.

Cores may be cut out as in other systems of boring, and extracted, so as to show the nature and dip of the strata. As the rope is flat, cores can be brought up without any twist.

The flat rope method is used by Messrs. Mather and Platt for holes from 20 inches to 45 inches in diameter. In the case of small holes from which no cores are required, they now adopt the American system on account of its expeditiousness.

From what has been said it is very evident that a great diversity of practice exists in making bore-holes, and the miner may have some difficulty in making up his mind which system to adopt for any given purpose. In the case of large undertakings, he usually applies to some firm of engineers, who by long and constant experience in their art are able to guarantee success.

Surveying Bore-holes. It is often assumed by boring engineers that the holes which they drill are perfectly vertical; but experience has shown that this is not always the case. It is, therefore, important to have some means of measuring the deviation of a bore-hole from the vertical, and surveying its exact course. A useful instrument for this purpose is Macgeorge's clinograph.* It consists in the main of two glass bulbs, the upper one carrying a plummet, the lower one a magnetic needle; both bulbs are filled with gelatine. When hot the gelatine is liquid, and the plummet and the needle are free to move; when the gelatine is cold both are set fast. The gelatine simply serves as a clamp which will act of itself after a certain time.

0

The exact construction is explained by Fig. 146.† The instrument consists of a cylinder terminating in a short neck and a bulb at the bottom. In this is a magnetic needle FIG. 146. attached to a hollow pear-shaped glass float, which will always stand upright upon its pivot and so enable the needle to swing round without touching the sides. A smaller glass cylinder, with a bulb at the top, is inserted through an air-tight cork and a brass capsule at the upper end of the large one. Its lower end passes into a cork, which prevents the escape of the float of the needle. The upper bulb contains a delicate plummet of glass, with diminutive hollow float at the top and a solid ball at the bottom, which is prevented from dropping out by a delicate grating. It is carefully adjusted to the specific gravity of the solidifying fluid which fills the cylinders and bulbs, and is so arranged that it will assume a vertical position whenever it is free to move.

"The Clinograph," Engineering, vol. xxxix. (1885), p. 260 Diamond Drill Clinometer," Min. Jour., vol. liii. (1883), p. 1509. + Brough, Mine Surveying, p. 276.

"The

In order to make use of these dip-recorders, or clinostats, as Mr. Macgeorge calls them, six are placed in a bath of warm water, which is heated nearly to boiling. In the meantime a brass cylinder is also heated by filling it several times with boiling water, and when the clinostats have been inserted one after the other into it, it is lowered into the bore-hole and allowed to remain there for two or three hours. By this time the gelatine will have set; the brass case is drawn up and the clinostats are examined one by one in a special instrument designed by Mr. Macgeorge. This has an arrangement for placing the clinostat in exactly the same position which it occupied in the bore-hole, and for enabling its angle of inclination and its magnetic bearing to be measured very accurately. The mean of the six sets of observations is then taken as representing the correct deviation.

If a bore-hole is approximately vertical, and the strata comparatively cool, the brass tube containing the clinostats may be lowered with a wire rope; but if the strata are hot or the borehole somewhat flat, 1-inch iron pipe is employed for inserting the brass case. Care is taken to interpose a distance tube of brass between the case and the pipes, to prevent their action on the magnets. If the bore-hole is warm, cold water is forced down the pipe so as to flow outside the case with the clinostats, and congeal the gelatine.

If observations are made at regular intervals, say at every 100 feet, the path of the bore-hole can be traced with great accuracy.

The apparatus may also be used over a core extractor when it is necessary to ascertain the direction and amount of the dip of the strata. Macgeorge employs a brass tube set excentrically, and provided with a bell-mouth below. This receives the end of the core, and the excentricity of the tube causes pressure on one side which makes the core break off. The core-extractor contains an inner tube, slotted from end to end, which expands as the core enters it and nips it tightly.

Mr. Macgeorge gives numerous instances of ascertained deflections of bore-holes. At Scotchman's United mine, Stawell, Victoria (Figs. 147 and 148), a bore-hole 370 feet deep, put down with a diamond drill, was found to have a deviation of 37 feet 3 inches It is calculated that £2311 would have been saved if the path of the drill had been surveyed before the driving was commenced. At the Oriental Company's mine a bore-hole turned out to be 60 feet 9 inches out of its proper course in a depth of 425 feet. Similar cases of deflection have been noted in bore-holes made in Germany both by the diamond drill, and by the percussive method.

The deviation from the vertical may likewise be recorded by Nolten's method, which depends upon the etching action of

* P. K., "The Deviation of Bore-holes," Colliery Guardian, vol. liii. (1887), p. 775.