Use of the Magnetic-Needle in Surveying Bore-holes. It has been assumed that the diamond drill always bores a perfectly straight hole, even though passing through rocks of different hardness. Actual experience reveals an entirely different state of things, the deviations sometimes being so great as to render a bore-hole misleading. An ingenious plan of correctly ascertaining these deviations has been devised by Mr. E. F. Macgeorge,* an Australian engineer. His plan consists in lowering into the bore-hole clear glass phials filled with a hot solution of gelatine, each containing, in suspension, a magnetic-needle, free to assume the meridian direction. The phials are encased in a brass protecting tube, and let down to the depth required, being allowed to remain for several hours until the gelatine has set.

It

The construction of the phials or clinostats can be seen from Fig. 96. The clinostat is a true cylinder of glass made to fit accurately within the brass guide-tube. At the lower end it terminates in a short neck and bulb, within which a magnetic-needle is so held by a glass float as to stand upright upon its pivot in every position of the phial, and thus allow the needle to assume the meridian freely without touching the sides of the bulb. Passed through an air-tight cork and screw-capsule at the upper end is a small glass tube terminating in another bulb above and with its open lower end inserted in a cork which enters the lower neck of the phial, thus preventing the escape of the needle and float in the lower bulb. The upper Fig. 96. bulb contains a very delicate plumb-rod of glass consisting of a fine rod terminating in a plumb of glass below and a diminutive bulbous float of hollow glass above. is carefully adjusted to the specific gravity of the gelatine in which it is immersed, so as to insure the rod being truly vertical whatever the position of the phial and bulb may be. When the gelatine is fluid the plummet hangs freely perpendicular, whilst the needle in the lower bulb assumes the magnetic meridian. When, however, the phial is at rest in any position, the contents solidify on cooling, and thus hold fast the indicating plummet and magnet in solid transparent material. On withdrawal from the bore-hole the phials can each be replaced at the same angle at which they cooled, and when the phial is revolved upon the part where the magnetic-needle is seen bedded in the gelatine, until the needle is again in the meridian, the phial is manifestly in the same direction, both as regards inclination and azimuth, as it was when its contents were congealed, and thus the gradient and bearings of the bore-hole can. * Engineering, vol. xxxix., 1885, pp. 260, 334.

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be determined. By repeating the observations at intervals of every 100 feet, the path of the bore-hole can be accurately mapped. The inclination and azimuth at the time of cooling is determined exactly by the recording instrument, or clinometer, which is a modification of the theodolite. The phial, with its congealed contents, is placed in a sheath of brass tubing (Fig. 97), attached

The

to a movable arm carrying the index of a vertical arc. upper bulb of the phial is brought into the field of two crossvisioned microscopes, carried with the arm round the vertical arc, which are kept truly in the same plane at every angle of inclination by a parallel motion. Upon the object-glass of each microscope vertical lines are drawn. The phial is revolved in its sheath, and the arm is moved along the arc by the tangent screw until the embedded plummet is made perpendicular from each point of view. The phial is now at the angle of inclination at which its contents solidified, and its lower bulb will be found nearly in the axis of the revolving arm, and about an inch above

the centre of a horizontal revolving circular mirror, having a system of parallel lines engraved across its face. Reflected in the mirror will be seen the image of the embedded needle, which, of course, pointed north before it was fixed by congelation in the bore-hole. If now the mirror is revolved until the number 270 of the graduated circle is opposite the marked end of the needle, and until the reflected image of the needle is parallel with the engraved lines, an index at the side of the graduated mirror frame will give the exact angle between the needle and the vertical plane of revolution of the phial, which is, in fact, the magnetic bearing of the inclined phial and of the bore-hole it occupied at the time of the application of the test.

This method was first applied at the Scotchman's United Mine at Stawell, in Victoria, and was so effectual as to enable the borehole to be found 37 feet away from its supposed position at a depth of 370 feet, a deflection that increased to the large amount of 75 feet at a depth of 500 feet. An exploratory level failed to find the bore-hole at its theoretical position, assuming the drill to have gone straight down. The subsequent search works lasted for more than a year, and cost altogether £3,663. Had the method been available at the commencement, the level driven would have cost only £1,352, and the saving effected would have been no less than £2,311.

Similar experiences have been met with in a number of other bore-holes in the mining districts of Victoria, and the consequence is that mine-proprietors are beginning to distrust the diamond drill altogether. Yet, if accurately surveyed, the most crooked bore is quite as useful as the straightest ever imagined by drill-makers. In view of these facts, the Victorian Government has contracted with the inventor to test all approved bores which have passed through auriferous rock.

By means of the clinograph, as the inventor terms his apparatus, a bore may be straightened when so deflected as to endanger the safety of the drill; for, suppose a bore-hole to have deflected suddenly, the depth of the point where the most serious deflection took place can be found. Then, if an india-rubber washer is forced down to 20 feet below this point, and liquid cement run in until it reaches some feet above the point of deflection, and allowed to set, then the drill may be again lowered and started gently, until it has started fairly in its corrected path, when the usual speed of boring may be resumed.

A less satisfactory method for ascertaining the inclination and direction of bore-holes was suggested by G. Nolten.* In the

* Preuss. Zeitschr., vol. xxviii., 1879, p. 176; Translation by C. Z. Bunning and J. K. Guthrie in Trans. . Eng. Inst. M.E., vol. xxix., p. 61.

instrument employed, the amount of deviation is etched upon glass by hydrofluoric acid; whilst its direction is found by means of a compass-needle, clamped by the aid of a stop-watch, after sufficient time has been allowed for settling. Notwithstanding the great imperfections of this instrument, its use in Germany has revealed some startling deviations of bore-holes. For example, in a bore-hole at Dienslaken, bored with a rotating drill, the deflection amounted to 47° at a depth of 750 feet. The borehole, undertaken by the German Government, at Lieth, in Holstein, was but little better than the preceding; but, by a lucky accident, the deflection of 3° at 984 feet gradually changed to the opposite quarter of the compass at 1,640 and 2,624 feet, and concluded with a deflection of only 1° at 3,280 feet.

Employment of a Powerful Magnet in Cases of Uncertain Holing. In 1846 Professor Borchers, of Clausthal, first proposed to employ a powerful magnet in cases of uncertain holing from one excavation to another. Since that date, he has improved the method in many ways, and has frequently employed it in practice with great success. In order to ensure a successful holing, the ends of the two levels must not be more than 20 yards apart. The apparatus employed consists of a powerful magnet, with a specially constructed protractor, a compass, and a small auxilary magnet.

The powerful magnet consists of six magnetised steel bars 4 feet in length, enclosed in two wooden boxes, provided with water-tight lids. One of these boxes contains only one magnet; whilst the other box contains the remaining five, separated from one another in the middle and at the ends by pieces of card-board. In the centre of its upper side, the larger box is provided with a pivot, which fits into an aperture in the smaller box. On this

pivot, the small box can be rotated, and the north pole of the magnet inside can thus be made to correspond with the south poles of the magnets in the large box. Consequently, a portion of the magnetic force of the latter is neutralised. The powerful magnet must be fixed in such a way that it can be pointed in various directions, without altering the position of its centre.

For this purpose, is employed a brass protractor, Figs. 98, 99, which can be screwed on to a thick board. At the centre of the protractor, a brass plate revolves, and between the turned-up edges of this, the principal magnet may be placed. At right angles to the longitudinal axis of the magnet, an index line is engraved. Provided the rock barrier is not more than 6 yards across, an ordinary compass, with a sensitive needle, may be employed. For greater distances, the compass-needle must be suspended by a silk fibre. Under these circumstances the steel pivot is removed from the compass, and a case screwed on to the plate, as shown in Fig 100. The sides are covered with glass to protect the needle from air currents. The upper end of the glass tube, containing the silk fibre, is provided with a contrivance for centering the needle. The latter is somewhat longer than the diameter of the compass dial, and does not require to be centered with mathematical accuracy, provided that both ends are read, and the mean of the two readings taken. The auxiliary magnet is a small magnetised bar 12 to 18 inches in length.

Fig. 100.

The mode of procedure is the following:At the end of one of the levels, the protractor is firmly fixed in such a way that its north and south line is in the magnetic meridian. At the end of the other level, the compass is set up, as nearly as possible at the same height as the protractor, and placed so that the needle indicates north. The needle must then be rendered astatic. To effect this, the auxiliary magnet is placed in the direction of the north and south line of the compass, on the side away from the principal magnet in the other excavation, and moved backwards and forwards until the force attracting the needle is neutralised. When these preparations are complete, the principal magnet, with the small box above it, is placed on the movable plate of the protractor, and brought approximately into the direction of holing. The powerful magnet then acts on the astatic needle of the compass, and causes it to take up a direction determined by a law enunciated by Gauss. But as the needle is not perfectly astatic for all directions, what attracting force remains must be again neutralised for the position taken up under the action of the powerful magnet. This is done by revolving on its pivot the small box above the principal magnet, until the north pole of the magnet it contains corresponds with the south poles of the other magnets. The action of the large magnet is thus